bootm: refactor do_reset and os boot function args

There is no need for each OS specific function to call do_reset() we
can just do it once in bootm. This means its feasible on an error for
the OS boot function to return.

Also, remove passing in cmd_tbl_t as its not needed by the OS boot
functions.  flag isn't currently used but might be in the future so
we left it alone.

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