This reverts commit 1a17e17c462bdd4e1d26d8742f8b7087273e04c2.

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Also, do not require -DRTOS_SDK and pick socket/low-level event manager
based on LWIP_SOCKET.

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LWIP part has been finally fully split from the ESP part, and ESP
platform defs reduced to bare minimum. It is now possible to use LWIP
in either low-level or socket mode without any ESP dependencies,
and NXP Kinetis platform is the first to make use of it.

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"#if FOO" still works with simple -DFOO, but gives more flexibility.
Specifically, if user expressed no preference (FOO is not defined),
we can apply reasonable defaults (this is the legitimate use of ifdef).

In short, from now on, please use

 #if MG_ENABLE_FOO

instead of

 #ifdef MG_ENABLE_FOO

Since we are all used to #ifdef, this change also adds a precommit check
to police this. Specifically, in *.h and *.c files that are Copyright Cesanta,
"ifdef" and "if defined()" are not allowed to be used with macros that contain
ENABLE or DISABLE, unless the like also contains "ifdef-ok".

Hence, if you are sure you want to use ifdef, use this:

 #ifdef MG_ENABLE_FOO /* ifdef-ok */

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Explicitly loosen them on the "big" platforms.

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