mtd: Make mtdparts work with pre-reloc env

The env in UBI needs to look up the mtd partition as part of relocation,
which happens before relocation.  Make the mtdparts code capable of
working on the default env to start with.

The code tries to set values in the env as well, but again, the env
isn't there yet, so add a check to setenv to not allow sets before the
env is relocated.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>