fastboot: add support for continue command

The fastboot continue command is defined to exit fastboot and continue
autoboot. This commit implements the continue command and the exiting of
fastboot only. Subsequent u-boot commands can be processed after exiting
fastboot. Autoboot should implement a boot script such as "fastboot; mmc
read <...>; bootm" to fully implement the fastboot continue function.

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
[TestHW: Exynos4412-Trats2]