In Linux, 'make' and 'make all' work equivalently; it builds both
vmlinux and modules.

In barebox, strangely, 'make all' builds both barebox and modules,
but 'make' builds only barebox because KBUILD_MODULES is unset.

The check for 'ifeq ($(MAKECMDGOALS),)' is missing since the day-one.

Fixes: b02e0966412c ("add additional linking stage for modules")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>