core+tools: support specifying F2FS as the fstype for userdata

- Support TARGET_USERIMAGES_USE_F2FS.
- Support BOARD_USERDATAIMAGE_FILE_SYSTEM_TYPE.
- Support "userdata_fs_type" in the prop dict.
- Update build_image to recognize f2fs and call the correct command.

Change-Id: If31cc8bd235f93a4c7814fab36e6e2d13d3037ad
Signed-off-by: JP Abgrall <jpa@google.com>