include: kernel-build: pass pkg-config overrides to kernel build

include: kernel-build: pass pkg-config overrides to kernel build

Pass suitable pkg-config overrides to the kernel build process in
order to let our pkg-config wrapper discover libraries provided
by tools/.

This mainly affects the use of libelf which is required for the
CONFIG_STACK_VALIDATION features. So far, the build system either
silently used host system libraries or kbuild simply disabled the
feature due to the lack of a suitable libelf.

Signed-off-by: Jo-Philipp Wich <[email protected]>