x86: use targets in the boot Makefile instead of CLEAN_FILES

Impact: cleanup

Instead of using CLEAN_FILES in arch/x86/Makefile, add generated files
to targets in arch/x86/boot/Makefile, so they will get naturally
cleaned up by "make clean".

Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>