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/Makefile.lib

@@ -99,11 +99,6 @@ __a_flags	= $(_a_flags)
 __cpp_flags     = $(_cpp_flags)
 else
 
-# 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)),$(o)))
-
 # -I$(obj) locates generated .h files
 # $(call addtree,-I$(obj)) locates .h files in srctree, from generated .c files
 #   and locates generated .h files