github: add action for OpenWrt base update (#3235)

Create a GitHub action to automatically create updates to the OpenWrt
base-version on manual run.

This allows to create base-modules updates where Gluon patches did not
change from the GitHub UI.

Skip PR creation. Automatically created PRs do not invoke subsequent
actions, thus not creating build-tests.

Signed-off-by: David Bauer <mail@david-bauer.net>