This review aims at introducing ubldr (loader with U-Boot interface) for MIPS
(32 and 64-bit, LE and BE).
The changes were tested with QEMU's 'mips' target.

Most of the implementation was lifted from the ARM version, the appropriate
MIPS-specific things were implemented.

With these changes I am able to go all the way through the u-boot->ubldr->kernel
boot chain in QEMU on all combinations of bit-ness and endian-ness.

For the tests I've used FAT32 disk images (as FAT32 is supported by U-boot),
which include /boot/kernel/kernel and /boot/kernel/ubldr.bin

In U-boot I do:
fatload ide 0 <LOAD_ADDR> /boot/kernel/ubldr.bin; go <LOAD_ADDR>
where LOAD_ADDR is 80800000 for 32-bit and ffffffff80800000 for 64-bit

Then it's the usual ubldr that takes over and loads and starts a kernel.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5313