boot2 tidyups:

(1) Use C globals for Miniboot argument preservation.
(2) For now disable future-facing "is it memory or a DTB pointer" logic
    for Miniboot.
/boot/loader improvements sufficient to get it booting FreeBSD -- hook up
64-bit ELF support, more debugging output (some to trim soon), usefully
preserve boot2 arguments and pass them on to the kernel, properly save
globals only after initialising bss.  We don't yet pass the full loader
environment on to the kernel on boot, but will soon.
Also build ELF support into MIPS boot loaders.