phase1,phase2: s/master/main for phase{1,2}

phase1,phase2: s/master/main for phase{1,2}

More and more projects are switching their repositories to use the
'main' branch instead of the 'master' branch. This also includes many
Linux upstream trees as well. Some trees are even removing their
'master' branches already.

I think this is becoming more and more mainstream and expected of
projects, so we should do the same.

References: https://openwrt.org/voting/2023-02-27
Signed-off-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Petr Štetiar <ynezz@true.cz> [commit facelift]