cmd_bootm.c: Make bootz handle BOOTM_STATE_FINDOTHER itself

As a zImage does not have a U-Boot header, we cannot really do what
BOOTM_STATE_FINDOTHER does, exactly.  Break the ramdisk/fdt portions of
bootm_find_other into bootm_find_ramdisk/fdt which can be called in both
cases.

Signed-off-by: Tom Rini <trini@ti.com>