kernel-yocto: do_kernel_configme: Fix silent sysroot poisoning error

kernel-yocto: do_kernel_configme: Fix silent sysroot poisoning error

do_kernel_configme calls merge_config.sh (installed in the sysroot by
the kern-tools-native recipe) which may invoke the compiler to complete
the configuration process.

Depending on the build (and dependencies), this may error due to sysroot poisoning [1].

The errors are similar to:

  make[1]: Entering directory '4.1+gitAUTOINC+a7e53ecc27-r0/linux-x64-standard-build' HOSTCC  scripts/basic/fixdep
  work-shared/x64/kernel-source/scripts/basic/fixdep.c:106:23: fatal error: sys/types.h: No such file or directory
  compilation terminated.
  make[2]: *** [work-shared/x64/kernel-source/scripts/basic/Makefile:22: scripts/basic/x86_64-nilrt-linux-fixdep] Error 1

Adding $TOOLCHAIN_OPTIONS to $CFLAGS before calling merge_configs.sh
fixes the error because $TOOLCHAIN_OPTIONS defines the sysroot and make
uses it to correctly compile & fill all missing kernel config options.

[1] http://lists.openembedded.org/pipermail/openembedded-core/2014-October/098253.html

Signed-off-by: Ioan-Adrian Ratiu <adrian.ratiu@ni.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>