[osboot] Add commandline parsing to gigaboot

It is expected that bootloader-specific args will be prefixed with
"bootloader.", though nothing enforces this or prevents gigaboot from
snooping on args intended for magenta.

As a first client of cmdline args, the netboot/localboot timeout can
be overriden with bootloader.timeout.

Change-Id: I1f5af10eef2ffde376f801fe8945f70b39366c9f
4 files changed