package/swupdate: bump version to 2020.04

package/swupdate: bump version to 2020.04

See full changelog : https://github.com/sbabic/swupdate/releases/tag/2020.04

Since commit
https://github.com/sbabic/swupdate/commit/82a157e35e9d01599e3c5818caa568899c17e6d2,swupdate
only supports using libubootenv to manipulate the U-Boot environment,
and no longer directly using the U-Boot tools, so we adjust the
Config.in help text and .mk logic accordingly.

Regenarated the default .config

Signed-off-by: Joris Offouga <offougajoris@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>