devtool: explicitly set main or master branches in upgrades when available

In particular this resolves devtool's inability to pick a branch
when the same tagged commit is avaiable in main and in a release maintenance
branch.

Thanks to Peter Kjellerstedt for the suggestion.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>