vold: Fix build for minivold and general cleanup

Change-Id: Ibc450dd86b77a0c4a25de08b2f07dfb9b2c70871
vold: Fix uses of board flags

Put LOCAL_CFLAGS += AFTER LOCAL_CFLAGS := ..
Else all flags will be removed/ignored ..
That fixes MAX_PARTITIONS flag usage ..

Change-Id: I96854b7de1eae6a235e8d31f7a9364a680b4f0b6