kbuild: do not try to build content of initramfs

When a file supplied via CONFIG_INITRAMFS pointed to a file
for which kbuild had a rule to compile it (foo.c => foo.o)
then kbuild would compile the file before adding the
file to the initramfs.

Teach make that files included in initramfs shall not be updated by adding
an 'empty command'. (See "Using Empty Commands" in info make).

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>