package/mender-update-modules: enable docker, rpm, and script modules

Starting with mender 5.x, the docker, rpm and script modules provided by
the mender package now reside in the mender-update-modules repository.

Even though the mender package provided by Buildroot is not updated yet to 5.x,
it is best to enable the modules here to help facilitate the future update of
the mender package to 5.x, and to ensure that any future modifications or bug
fixes to these modules are easy to apply by simply bumping the upstream package
version.

Script is enabled by default to preserve the existing behavior of the mender
package.

Signed-off-by: Adam Duskett <adam.duskett@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>