kbuild: skip 'addtree' and 'flags' magic for external module build

Backport from kernel
commit 58156ba4468f ("kbuild: skip 'addtree' and 'flags' magic for external module build")

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

scripts/Makefile.host

@@ -72,13 +72,15 @@ _hostc_flags   = $(KBUILD_HOSTCFLAGS)   $(HOST_EXTRACFLAGS)   \
 _hostcxx_flags = $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
                  $(HOSTCXXFLAGS_$(basetarget).o)
 
-ifeq ($(KBUILD_SRC),)
 __hostc_flags	= $(_hostc_flags)
 __hostcxx_flags	= $(_hostcxx_flags)
-else
+
+ifeq ($(KBUILD_EXTMOD),)
+ifneq ($(KBUILD_SRC),)
 __hostc_flags	= -I$(obj) $(call flags,_hostc_flags)
 __hostcxx_flags	= -I$(obj) $(call flags,_hostcxx_flags)
 endif
+endif
 
 hostc_flags    = -Wp,-MD,$(depfile) $(__hostc_flags)
 hostcxx_flags  = -Wp,-MD,$(depfile) $(__hostcxx_flags)

scripts/Makefile.lib

@@ -143,15 +143,14 @@ _c_flags += $(if $(patsubst n%,, \
 		$(CFLAGS_KASAN))
 endif
 
-# If building the kernel in a separate objtree expand all occurrences
-# of -Idir to -I$(srctree)/dir except for absolute paths (starting with '/').
-
-ifeq ($(KBUILD_SRC),)
 __c_flags	= $(_c_flags)
 __a_flags	= $(_a_flags)
 __cpp_flags     = $(_cpp_flags)
-else
 
+# If building the kernel in a separate objtree expand all occurrences
+# of -Idir to -I$(srctree)/dir except for absolute paths (starting with '/').
+ifeq ($(KBUILD_EXTMOD),)
+ifneq ($(KBUILD_SRC),)
 # -I$(obj) locates generated .h files
 # $(call addtree,-I$(obj)) locates .h files in srctree, from generated .c files
 #   and locates generated .h files
@@ -161,6 +160,7 @@ __c_flags	= $(if $(obj),$(call addtree,-I$(src)) -I$(obj)) \
 __a_flags	= $(call flags,_a_flags)
 __cpp_flags     = $(call flags,_cpp_flags)
 endif
+endif
 
 # Modified for U-Boot: LINUXINCLUDE -> UBOOTINCLUDE
 c_flags        = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(UBOOTINCLUDE)     \