bootm: Add subcommands

Add the ability to break the steps of the bootm command into several
subcommands: start, loados, ramdisk, fdt, bdt, cmdline, prep, go.

This allows us to do things like manipulate device trees before
they are passed to a booting kernel or setup memory for a secondary
core in multicore situations.

Not all OS types support all subcommands (currently only start, loados,
ramdisk, fdt, and go are supported).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>