Streamling UART code and add failsafe functionality

- UART now has a codepath for automatically resetting on reception
of a NULL byte if the flag is enabled
- UART also now heartbeat echos the character 255 whenever it
receives 255, if the flag is enabled

This means that we can positively identify the presence of a FSU
on the serial bus and reset it if is acting abnormally.
Add base multi_sensor_module code.  It's just a clone of pv_module with the names changes.
Add heartbeat functionality
Bug fix
Bug fix
Add heartbeat, reset and attached commands to FSU

FSU now supports queries on its status and will reset itself on
command to remove any potential error conditions.  FSU also checks
to make sure a MoMo is attached before sending an RPC, removing a
potential case where it could loop forever sending an RPC into an
emty wire.
Add support for fsu reset,attached and heartbeat to modtool
Working skeleton code for multi_sensor_module

- added line in build_settings.json to define the chip for the module
- ripped out all sdcard, fat32 and SPI code
- updated MIB file to match module name
Rename headers and remove chaff
Merge branch 'feature-robustify-fsu' into upstream/multisensor
Merged FSU changes for robust operation and added working
sensor code.
Update reflashing to not call bus_reset
Merge branch 'pr-support-pic24FJ64GA306' into upstream/multisensor
Fix integer overflow in pic12_executive on 16lf1847

When using the entire RAM (8192 words), there are 256 rows
which overflows the 8-bit variable used to store the current
row, causing the chip to rewrite itself in circles.  Change
the logic to compare against row-1 so that it saturates at 255.
Working code to use digitally controllable op-amp.
Add reset controller feature to modtool
Add proxy module for multisensor board to pymomo.commander
Add multisensor test program
Add gainsetting ability
Fix gain2 setting