* Add set_reboot_order API and config.txt properties
If set_reboot_order is defined in config.txt or set via vcmailbox
then this will override the bootloader config BOOT_ORDER property
on the next reboot. The parameter is stored in a reset safe register
and is cleared by the bootloader after reading it.
Typically, the config.txt value only be used via rpiboot to
override the boot-order on the next reboot. Otherwise, it should
reside in a conditional section so that the boot order is not
overridden on every reboot.
Example, test network boot
sudo vcmailbox 0x0003808b 4 4 0xf4612; sudo reboot
