Falcon mode boots the kernel directly from SPL, without loading
the full U-Boot.

As pico-imx7d does not have a GPIO for selecting Falcon versus
normal mode, enter in Falcon mode when the customer selects
the CONFIG_SPL_OS_BOOT option in menuconfig.
Signed-off-by: NFabio Estevam <festevam@gmail.com>
Signed-off-by: NOtavio Salvador <otavio@ossystems.com.br>