Set up Vold switchable pair to allow swapping internal/external SD

Uncomment the line persist.sys.vold.switchexternal=1 in bulid prop
to use

Change-Id: Ic669d2682f9d9ee2400693d987610528fb653e7a
Use shared defconfig

Change-Id: I3ccad7ec6bdb715e31ab4677bf63196ef302e696

Conflicts:
	BoardConfig.mk