Flashconfig

This implements a new FlashConfigSource that uses the last two blocks from the end of the internal 512k of flash rom. It also enhances config-get and config-set feedback and fixes some bugs in FileConfigSource.

It also adds the following commands to SimpleShell:
mcu: returns the microcontroller type ID which tells the difference between 1768 and 1769.
serial: returns the microcontroller serial number which is unique to each chip
isp: reboots into isp mode for serial programming of a bootloader via uart