Make RunMode.BOOTLOADER work on imxrt

[jepler]

Right now it just restarts CircuitPython:

>>> microcontroller.on_next_reset(microcontroller.RunMode.BOOTLOADER)
>>> microcontroller.reset()

[tio 15:51:33] Disconnected
[tio 15:51:35] Connected
Auto-reload is on. Simply save files over USB to run them or enter REPL to disable.

Press any key to enter the REPL. Use CTRL-D to reload.

Adafruit CircuitPython 6.2.0-beta.3-258-gc0d4b3732-dirty on 2021-03-17; IMXRT1010-EVK with IMXRT1011DAE5A