cmd: Split 'bootz' and 'booti' out from 'bootm'

The bootz and booti commands rely on common functionality that is found
in common/bootm.c and common/bootm_os.c.  They do not however rely on
the rest of cmd/bootm.c to be implemented so split them into their own
files.  Have various Makefiles include the required infrastructure for
CONFIG_CMD_BOOT[IZ] as well as CONFIG_CMD_BOOTM.  Move the declaration
of 'images' over to common/bootm.c.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Tom Rini <trini@konsulko.com>