buildsys: Teach main Makefile howto build pl/sunxi-spl.bin

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Henrik Nordstrom <henrik@henriknordstrom.net>

Conflicts:
	Makefile
sunxi: Really fix case-insensitive *_config targets

Conflicts:
	Makefile
Fix sunxi compiler warnings

Fixes two sunxi related compiler warnings

Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>