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>