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
Merge pull request #1 from CyanogenMod/cm-12.0

vold: Respect board flags in minivold