binaries-marvell: bump version and switch to common firmware

binaries-marvell: bump version and switch to common firmware

Since release 18.12 binaries-marvell repository provides
common firmware supporting both A7K and A8K SoC families.

This commit bumps package version to 18.12 and removes
platform specific binary selections from Config.in.
Single firmware image suitable for both A7K and A8K
platforms is now specified in mk file explicitely.

Legacy handling is not needed, as configs which did have
the option set will continue to work without change.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
[Arnout:
 - Remove BINARIES_MARVELL_IMAGE entirely;
 - Add remark about legacy handling;
 - Remove the deprecated option from the defconfigs and test that use
   it.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>