Add support for 'fastboot reboot-bootloader'

- Add calls to get and set the MTK_WDT_NONRST2 register, which isn't
cleared on a reset.
- Implement fastboot_set_reboot_flag, using the MTK_WDT_NONRST2 register
to store the flag value at reset time
- Enable MISC_INIT_R and use it to handle the flag, board_init runs before watchdog startup.

Change-Id: I3c18ab3d97b4681775d426d487bf95968ab943d4