gluon-autoupdater: split CONFIG_GLUON_BRANCH into two separate options

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.