toradex: configblock: initialize MMC before switching partition

If the module is in serial downloader mode, we do no longer read the
environment from eMMC. Therefor, the eMMC is unitialized when trying
to read the config block. Use mmc_init to initialize the selected
MMC device before using it.

Note: In case the MMC has already been initialized, the mmc_init
detects that and returns immediately.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>