Split internal and external sdcards

Two new types are introduced:
sdcard_internal
sdcard_external

The existing type of sdcard, is dropped and a new attribute
sdcard_type is introduced.

The boolean app_sdcard_rw has also been changed to allow for
controlling untrusted_app domain to use the internal and external
sdcards.

Change-Id: Ic7252a8e1703a43cb496413809d01cc6cacba8f5