Add software method to enter bootloader

[bessman]

This PR makes it possible to enter bootloader mode via software, without pressing the BOOT button. By sending the magic number 0xDECAFBAD to the PSLab soon after reboot (within about 500 ms), the PSLab will stay in bootloader mode. This is useful for several reasons:

• During firmware development, it's annoying to have to press the BOOT and RESET buttons all the time
• The PSLab v5 has not buttons, so entering bootloader currently requires short-circuiting two pins on the MCU
• Being able to drop to bootloader with a command makes it possible to do firmware development remotely, e.g. by SSH:ing into a remote machine connected to a PSLab