Rather than have a long and if check in the Makefile, mark the default
lowlevel_init function as weak (as we do on armv8) so that SoCs can
override it if needed, and it will still be discarded if unused.
Provide a weak s_init as well to allow for this to link and be
discarded.

Signed-off-by: Tom Rini <trini@konsulko.com>