Add a uImage build target for MIPS, which builds uImage.gz (a U-Boot
image of vmlinux.bin.gz), and then symlinks it to uImage. This allows
for the use of other compression algorithms in future, and is how a few
other architectures do it.
It's enabled conditionally on load-y >= 0xffffffff80000000 which
hopefully allows 64bit kernels to also work as long as the load and
entry address can be represented by the 32bit addresses in the U-Boot
image format.