Lock picotool version on master to 2.1.1

[will-v-pi]

The master SDK branch (and the SDK release tags eg 2.1.1) should point to specific picotool tags, to ensure you get a compatible picotool - currently it points to the develop branch of picotool

This has been set to develop for the 2.1.0 and 2.1.1 releases, so those releases may break in the future if picotool loses backwards compatibility (eg SDK header movement, or changes to picotool commands)