vold: Mount ext4/f2fs sdcards with sdcard_posix context

Change-Id: I05d9b57cc28ffd1f8deb8148d81c7f6fad0aa8db
vold: Respect board flags in minivold

A few board flags are relevant to vold and minivold. Add them to
common_cflags so that they get used in both.

Change-Id: I585f67024d77c83a2df5d0f9733665ebed2e77b7