libbootloader_message: use different fstab paths for normal/recovery boot

libbootloader_message is used by both normal boot and recovery boot.
It needs to use different fstab paths, respectively. Otherwise, factory
reset will fail when we move /fstab.{ro.hardware} to /vendor/etc/.

  Recovery boot: fs_mgr_read_fstab_with_dt("/etc/recovery.fstab")
  Normal boot: fs_mgr_read_fstab_default()

Bug: 35811655
Bug: 36502022

Test: normal boot sailfish, go to Settings > System & tap on
      "Factory Data reset"
Test: recovery boot sailfish

Change-Id: I253f5bdfb9be8a01f80856eb1194f85cdf992bbd