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: Ie0f80ba3efd2f4a64d1cacd36299bcc7012ee9e9
Use shared defconfig and variant defconfig for 512MB

Change-Id: Iaeff9476caf34f84fd8ee8f5838cb402efaa02b0