Tweak 5.0 L decrypt

Mount the vendor partition if it exists so we can use any
proprietary files we may need.
Relocate auto decrypt when default_password is in use to after all
partitions are added so that we can mount the vendor partition.

Change-Id: I93455a35695779f53ef57a82d3d45c7216c13639
Add flag for including additional files built from source in TWRP

Some devices may need additional binaries or libs that TWRP
normally does not need included in /sbin
Use TW_RECOVERY_ADDITIONAL_RELINK_FILES to build them from source
and get them added to the recovery ramdisk.
For example, Nexus 9 needs libc++ included in order to decrypt
the data partition. The following will build it from source and
include it in TWRP:
TARGET_RECOVERY_DEVICE_MODULES := libc++
TW_RECOVERY_ADDITIONAL_RELINK_FILES := \
    out/target/product/volantis/system/lib/libc++.so

Change-Id: I147c0d5569c83514d2e50226ecc50e80bf8aac36
Add /firmware to the mount list for L decrypt

Needed by Nexus 6 and possibly others

Change-Id: I3309e2141a21a94dce90e34cced16450b591a92c
Fix process to disable stock recovery flashing

/system/etc/install-recovery.sh no longer exists on AOSP devices
so we will stop checking for it.

Change-Id: I9074dfc443371a27b909faa8f66998683cfb6171
Properly set tw_settings_path after decrypt on data media devices

When tw_settings_path is not set correctly for /data/media/0 when
present, custom themes will not load (and who knows what else may
also be broken) so set it properly after decrypting the device.

Change-Id: Id3dff04f62cf9c953c8a2ca07fe9ac597dcda20f