env: mmc: statically set the environment partition name

The new opt-out setting, CONFIG_ENV_MMC_PARTITION, statically sets
the MMC environment partition name. Prior to this patch, the only way
to declare this partition name was by creating a
'u-boot,mmc-env-partition' parameter in the device-tree's /config node.

This setting provides additional flexibility, particularly in cases
where accessing the device-tree is not straightforward (e.g. QEMU).

If undeclared, the device-tree's setting will be used.

Signed-off-by: Emmanuel Di Fede <emmanuel.difede@cysec.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>