MFC r264605:

Based on xlp_machdep.c and completed the list of options based on
boot/mips/beri/loader/metadata.c allow FDT configuration to set
command line options.
This leads to an interesting quesiton of future interactions with loader.
However for configurations without loader this allows bootverbose or boot
single user to be set by compiling a new kernel, which is good enough for
testing and debugging.

Reviewed by: rwatson
Sponsored by: DARPA/AFRL