fs_mgr: Add fs_mgr_flag overlayfs_remove_missing_lowerdir

If this flag is given, then fs_mgr_mount_overlayfs_fstab_entry() shall
filter out missing directories in the lowerdir= list.

For example,

  test /mnt/vendor/overlay_test overlay \
    ro,lowerdir=/dir1:/dir2:/missing_dir3 \
    first_stage_mount,overlayfs_remove_missing_lowerdir

should mount the overlayfs device with "lowerdir=/dir1:/dir2".

Bug: 186342252
Test: Manual boot test with modified fstab on CF
Change-Id: Id06b37d0c236528cef981e495280b4f4e9c2b4bb