Command ID sign mismatch

[kierdavis]

Command IDs sent to the Arduino which are greater than 32767 will be returned as negative values instead (i.e. Arduino is implicitly casting an unsigned 16-bit int to a signed 16-bit int).

robotd log:

: Sending to servo assembly: b'@64015 gpio-read 10\n'
: Got back from servo: b'# @64015 gpio-read 10\n'
: Got back from servo: b'@-1521 > low\r\n'
: Got back from servo: b'@-1521 + OK\r\n'

robotd works around this because it bitwise ANDs returned command IDs with 0xFFFF before examining them, but it would be nice if servo-firmware didn't do this in the first place.