fs_mgr: Allow overriding the super partition name via the kernel commandline.

For retrofit devices where the super partition is not called "super",
this allows the correct partition to be passed via the kernel
command-line, i.e.:

  androidboot.super_partition=system

Since retrofitting is only supported for A/B devices, we assume that
a partition named this way must be slot-suffixed.

Bug: 116802789
Test: manual test
Change-Id: I930dbbd397d5552e9ded89baa5a7bb0e63c67426