cmake: do not look into native sysroot in the nativesdk environment

cmake: do not look into native sysroot in the nativesdk environment

I am not sure why we do this in the first place, but it is causing
cmake to erroneously pick up items from the native sysroot
when building for the target and the target item is missing, for example:

https://autobuilder.yoctoproject.org/typhoon/#/builders/59/builds/198/steps/7/logs/step2c

Note that for executable programs this variable is not referred to,
as set by
 set( CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER )
in the same file.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>