kbuild: enable use of AFLAGS and CFLAGS on commandline

The previous patches was preparation.
With this patch we can now say:
make CFLAGS=-Os vmlinux

And the option specified will be appended to the
options passed to gcc for C files.

For assembler use:
make AFLAGS=-foo vmlinux
for the same functionality.

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