CL: Make MG_CTL_MSG_MESSAGE_SIZE overridable

Fix https://github.com/cesanta/mongoose/pull/987

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CL: Add mbuf_append_and_free() and mbuf_move()

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It can specify how much data was actually processed and the rest will be re-delivered on next poll.

CL: mg: Add ability for multipart data handler to provide pushback

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Replacement for fw_meta.py commands and also supports unbundling .hex files.

General syntax is:

```
 $ mos create-fw-bundle -o fw.zip part1:prop1=val1,prop2=val2,... part2:...
```

Example of building a mos fw bundle for ESP8266:
```
 $ mos create-fw-bundle -o fw.zip --platform esp8266 --build-info=build_info.json \
    boot:addr=0x0,src=rboot.bin,update=false \
    boot_cfg:addr=0x7000,size=0x1000,fill=0xff \
    fw:addr=0x100000,src=demo-c.bin \
    fs:src=fs.bin,type=fs,addr=0x8000,fs_size=262144,fs_block_size=4096,fs_page_size=256,fs_erase_size=4096
```

Example of converting a HEX file to a bundle:
```
 $ mos create-fw-bundle -o fw.zip host:type=host,src=hostfw.hex
```

CL: mos: Add create-fw-bundle command

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CL: none

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CL: mg: Read all available data during recv poll

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CL: Add MG_MK_STR_N() and mg_str_starts_with()

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Should use length of the exected response, not the response sent by the user.

CL: mg: Fix digest auth result comparison

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CL: none

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CL: none

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CL: Update mbedTLS to 2.13.1

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CL: mg: Check HTTP chunk size, ensure it's reasonable

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Clean the HTTP connection state when ia request/response has been fully
buffered and handler invoked.

Fixes https://github.com/cesanta/mongoose/issues/971

CL: mg: Fix handling of keepalive HTTP requests/responses

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Go back to Ubuntu 16.04 and older gcc-arm (4.9.3).

Boot loader just won't run when compiled with newer GCC (7.x).
It's likely that it can be fixed with compile switches but I couldn't figure it out quickly, so going to just downgrade for now to get it back to workign state.

CL: CC3200: Fix boot loader regression

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CL: Add mgos_gpio_blink()

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mbedTLS is just another lib now

CL: Use mbedTLS from mongoose-os-libs/mbedtls

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CL: Integrate https://github.com/cesanta/mongoose/pull/667, fix documentation

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CL: none

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CL: Docs nits

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 * Update CubeL4 to 1.13.0
 * Rebuild OurTLS with `-DMBEDTLS_X509_CA_CHAIN_ON_DISK`
 * Add `-Wextra` and fix build issues

CL: Update STM32 build image

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Listeners still TODO

CL: ISM43XXX networking support (TCP, UDP client)

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CL: STM32: Make LWIP optional

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Can be used for cases where device has no networking but mongoose is still needed for its event loop.

CL: mg: Add net_if_null, a no-op net interface

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 * Change return type of mg_mgr_poll to return number of events
 * Add mg_mgr_min_timer
 * Refactor main poll loop to remove LwIP-specific stuff

CL: Refactor mg polling

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CL: Fix https://github.com/cesanta/mongoose/issues/962 - file upload on windows

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A smattering of other related changes

CL: STM32 OTA support for L475-IOT01 and DISCO-F746

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CL: Mongoose 6.12

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h/t @M4GNV5

Fixes https://github.com/cesanta/mongoose/issues/943
Closes https://github.com/cesanta/mongoose/issues/944

CL: mg: Don't touch send_mbuf when sending MQTT messages

PUBLISHED_FROM=da7b4f8acec2e403caa4addd5739d036a6a74c76

Fixes https://nvd.nist.gov/vuln/detail/CVE-2018-10945

CL: mg: Fix body length calculation in mg_handle_cgi

PUBLISHED_FROM=0c30cf36fdb67c75f6148468701e23d6ee72d953

Closes https://github.com/cesanta/mongoose/pull/955

CL: mg: Add host name verification for OpenSSL

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CL: Make esp8266 compilation a little less verbose

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Closes https://github.com/cesanta/mongoose/pull/945

CL: Fix build with CS_ENABLE_STDIO=0

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CL: Add mg_strstrip: trims whitespace at the ends of s

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Use vararg macros for this.

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