An invalid branch may be set for various reasons:
- Previous firmware had an invalid default branch
- Branch list has changed and old UCI branch config was removed by a
site-specific upgrade script
- Manual UCI configuration
The new options are CONFIG_GLUON_AUTOUPDATER_BRANCH and
CONFIG_GLUON_AUTOUPDATER_ENABLED and allow to control the default branch
and default enable status separately.
The `or ''` fallback in targets/generic is removed, as GLUON_ENV will
set all variables in GLUON_VARS, making previously non-existing
variables exist with an empty value.
