Add F2FS support for apexer

The new flag --payload_fs_type enables the user to choose between ext4
and f2fs for apex_payload.img

Test: ./runtests.sh

Bug: 158453869
Change-Id: I2858d834924c5cec0f809c4f5c8b2e622b8fdf47
(cherry picked from commit 856d59d5dace3179b836e3b4bfb1f4c9bf4d653e)
4 files changed