kbuild: refactor Makefile.asm-generic

 - Use conventional $(MAKE) $(asm-generic)=<dir> style
   for directory descending

 - Remove unneeded FORCE since "all" is a phony target

 - Remove unneeded "_dummy :=" assignment

 - Skip $(shell mkdir ...) when headers exist in the directory

 - Misc cleanups

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>