kbuild: add forward declaration of default target to Makefile.asm-generic

$(kbuild-file) and Kbuild.include are included before the default
target "all".

We will add a target into Kbuild.include.  In advance, add a forward
declaration of the default target.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>

scripts/Makefile.asm-generic

@@ -5,6 +5,9 @@
 # and for each file listed in this file with generic-y creates
 # a small wrapper file in $(obj) (arch/$(SRCARCH)/include/generated/$(src))
 
+PHONY := all
+all:
+
 kbuild-file := $(srctree)/arch/$(SRCARCH)/include/$(src)/Kbuild
 -include $(kbuild-file)