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 <jo@mein.io>
(cherry picked from commit fe43969336201f2cc7d103b68fd6e65989bee184)

include/kernel-build.mk

@@ -131,10 +131,16 @@ define BuildKernel
 	$(Kernel/Configure)
 	touch $$@
 
+  $(LINUX_DIR)/.modules: export STAGING_PREFIX=$$(STAGING_DIR_HOST)
+  $(LINUX_DIR)/.modules: export PKG_CONFIG_PATH=$$(STAGING_DIR_HOST)/lib/pkgconfig
+  $(LINUX_DIR)/.modules: export PKG_CONFIG_LIBDIR=$$(STAGING_DIR_HOST)/lib/pkgconfig
   $(LINUX_DIR)/.modules: $(STAMP_CONFIGURED) $(LINUX_DIR)/.config FORCE
 	$(Kernel/CompileModules)
 	touch $$@
 
+  $(LINUX_DIR)/.image: export STAGING_PREFIX=$$(STAGING_DIR_HOST)
+  $(LINUX_DIR)/.image: export PKG_CONFIG_PATH=$$(STAGING_DIR_HOST)/lib/pkgconfig
+  $(LINUX_DIR)/.image: export PKG_CONFIG_LIBDIR=$$(STAGING_DIR_HOST)/lib/pkgconfig
   $(LINUX_DIR)/.image: $(STAMP_CONFIGURED) $(if $(CONFIG_STRIP_KERNEL_EXPORTS),$(KERNEL_BUILD_DIR)/symtab.h) FORCE
 	$(Kernel/CompileImage)
 	$(Kernel/CollectDebug)