kbuild: properly pass options to hostcc when doing make O=..

This fix a longstanding bug where proper options was not
passed to hostcc in case of a make O=.. build.
This bug showed up in (not yet merged) klibc, and is not known
to have any counterpart in-kernel.
Fixed by moving the flags macro to Kbuild.include so it can be used
by both Makefile.lib and Makefile.host.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/Kbuild.include

@@ -87,6 +87,11 @@ cc-ifversion = $(shell if [ $(call cc-version, $(CC)) $(1) $(2) ]; then \
 # $(Q)$(MAKE) $(build)=dir
 build := -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.build obj
 
+# Prefix -I with $(srctree) if it is not an absolute path
+addtree = $(if $(filter-out -I/%,$(1)),$(patsubst -I%,-I$(srctree)/%,$(1))) $(1)
+# Find all -I options and call addtree
+flags = $(foreach o,$($(1)),$(if $(filter -I%,$(o)),$(call addtree,$(o)),$(o)))
+
 # If quiet is set, only print short version of command
 cmd = @$(echo-cmd) $(cmd_$(1))