build_image: Set file system labels on disk images

When building images set the mount point (data,cache,system) as the
filesystem label on the generated image.  This is required for the
Android Emulator.

IMPORTANT: This depends on the following patch to system/extras/:

    https://android-review.googlesource.com/#/c/120047/



which has already been submitted to AOSP and merged into the
internal tree.

Change-Id: Iaeb45462570c26d37980f1cc5d8a1b929e476c89
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>