Add support to `tools/metrics.py` to compute the size delta of mpy-cross,
alongside the sizes of port firmware. This is an easy and cheap addition
because mpy-cross is usually built before the ports are.
Although the size of mpy-cross is not critical, it's still a nice
indication of how changes affect code size, and helps to eliminate any
unwanted increases in mpy-cross.
Signed-off-by: Damien George <damien@micropython.org>
0d9aafd9f5