Add BBU configuration to allow programming Barebox to eMMC. Normally
factory U-Boot is shipped in mmc0.boot0, so us using user partition
for Barebox should allow those two to coexist.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>