Now that the elf file is loaded by the bootm core, there is no need for
elf pointer anymore. Thus all elf related fields can be removed and
bootm_load_os can be used.

Signed-off-by: Clement Leger <cleger@kalray.eu>
Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>