fastboot: add support for reboot-bootloader command

The "fastboot reboot-bootloader" command is defined to
re-enter into fastboot mode after rebooting into
bootloader. This command is usually used after updating
bootloader via fastboot.

This commit implements only a generic side of the
command - setting of the reset flag and then resetting.
Setting of the reset flag is implemented using __weak
fb_set_reboot_flag() function. The actual setting and
checking of the reset flag should be implemented by
a boot script and/or board/SoC specific code.

Signed-off-by: Alexey Firago <alexey_firago@mentor.com>
Tested-by: Steve Rae <srae@broadcom.com>
[Test HW: bcm28155_ap board]