bmips: Build U-Boot into the XG6846 target

bmips: Build U-Boot into the XG6846 target

It appears that the CFE boot loader found in the XG6846
cannot load kernels over a certain size, and the old
relocate hack is not working.

What to do? We can build a small U-Boot into the image,
make CFE boot that, place the kernel immediately after
U-Boot, and use U-Boot to boot the system instead.

The compiled u-boot.bin becomes around ~300KB and with
LZMA compression it will swiftly fit into 128KB, so
we use two 64KB erase blocks right after the CFE to
store an imagetag:ed U-Boot.

Reviewed-by: Paul Donald <newtwen+github@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>