Makefile: add target to produce a SPL compatible uImage
This is mostly useful during the initial port of barebox to a new
board which is supported by u-boot. It also allows starting barebox
from a SRAM-based u-boot SPL.
A different load address can be set like this:
make barebox.uimage UIMAGE_BASE=0x80000000
Also add barebox.ubl to CLEAN_FILES.
Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>