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>

fs_mgr/fs_mgr_slotselect.cpp

@@ -35,6 +35,8 @@ int fs_mgr_update_for_slotselect(struct fstab *fstab)
                   return -1;
                 }
                 got_suffix = 1;
+                // remove below line when bootloaders fix androidboot.slot_suffix param
+                if (suffix[0] == '_') suffix.erase(suffix.begin());
             }
 
             if (asprintf(&tmp, "%s_%s", fstab->recs[n].blk_device, suffix.c_str()) > 0) {