If picotool is not installed, it's fetched and built when compiling each
rp2 board. And the "develop" branch of picotool is used instead of a
release. Installing it manually using the "master" branch means the latest
released version is used (instead of a possibly unstable development
version), and also makes building each rp2 board a little faster.
Signed-off-by: Damien George <damien@micropython.org>