This will allow to override CUDA CMake variables from first CMake call,
like cmake -DCUDA_TOOLKIT_ROOT_DIR=... -DCUDA_CUDA_LIBRARY=...

do not unset variables if CUDA_TOOLKIT_TARGET_DIR is not defined

it is marked as obsolete

* improve `CUDA_TARGET_CPU_ARCH` cache initialization,
    allow to override initial value from calling script;
* add `CUDA_TARGET_OS_VARIANT` option to select OS variant;
* add `CUDA_TARGET_TRIPLET` option to select target triplet from
    `${CUDA_TOOLKIT_ROOT_DIR}/targets` folder;
* remove `CUDA_TOOLKIT_TARGET_DIR` option, now it is calculated from
    `CUDA_TARGET_TRIPLET`, the old approach still can be used for compatibility;
* for CUDA 6.5 and newer try to locate static libraries too, because
   in 6.5 toolkit for ARM cross compilation only static libraries are included.

use targets/armv7-linux-androideabi is exists