fs_mgr: allow using legacy A/B suffix

Right now all platforms supporting A/B have not fixed
androidboot.slot_suffix kernel command line parameter. To avoid breaking
all those platform allow using legacy support bootloaders get fixed.

Bug: 36533366
Test: boot
Change-Id: I0ad349f262d0b7f4897f3e184cb053b1440b2d5b
Signed-off-by: Fernando Lugo <flugo@google.com>