doc: uefi.rst: Document launching UEFI binaries from FIT images

This patch adds a new section "Launching a UEFI binary from a FIT image"
documenting the usage of the CONFIG_BOOTM_EFI extension to bootm command
that offers a verified boot alternative for UEFI binaries such as GRUB2.

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