The TARGET_RW_FILE_SYSTEM_TYPE will contain the rw filesystem
to use. The rw filesystem for the devices can be f2fs or ext4.
If TARGET_RW_FILE_SYSTEM_TYPE is not defined, the default
fs will be f2fs.
Bug: 356425337
Test: $ source build/envsetup.sh
$ lunch [zumapro]-trunk_staging-userdebug
$ m
$ ./vendor/google/tools/flashall -w
Flag: EXEMPT bugfix
Change-Id: Ib087a06a98525ca449253dffce9df470a05432d3