Partition: Fix parsing of several twrp fstab flags

fix parsing when using storagename=, backupname= and userdataencryptbackup= in twrp.fstab
also split encryptable= and forceencrypt= into seperate blocks
and make storage=Y working

Change-Id: I674f7ffe7f9e4ff121c4dccf79096360e044535c