fs_mgr_fstab: Parse overlayfs options from fs flags

Parse the "lowerdir=" option from fs mount flags instead of fs_mgr flags
for consistency.

Before:

  none /mnt/product/test1 overlay ro lowerdir=/1:/2,first_stage_mount

After:

  none /mnt/product/test1 overlay ro,lowerdir=/1:/2 first_stage_mount

Bug: 188862155
Test: Boot to normal with overlayfs mount entries in first stage fstab
Change-Id: I6d6abd44ab32afadec428005f4aece834f9c8905