Remove bootloaders section and bootloaders
Only MIPS had any information and only for COLO.
There's many many different bootloaders and methods for getting to the
point where Linux can run depending on arch, board, and user desires.
Later, we'll move a quick bit of info about picking a bootloader to the
"bootable" section after building the kernel.
Trying to document how to build every possible, even just the popular,
bootloaders, isn't reasonable.