bootm: Add a bootm command for type IH_OS_EFI

Add support for booting EFI binaries contained in FIT images.
A typical usage scenario is chain-loading GRUB2 in a verified
boot environment.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>