bootmeth_cros: Require bootm.o and bootm_os.o

In order to use bootmeth_cros, at least on non-X86, we need to be able
to start any type of kernel that the "bootm" code paths can handle.  Add
these objects to the required list for this option.

Signed-off-by: Tom Rini <trini@konsulko.com>
---
Cc: Simon Glass <sjg@chromium.org>