factor core.mk product definition

Split core.mk, which is currently set up for phones and tablets, into
a core.mk and core_minimal.mk, so that non-phone devices can inherit
from core_minimal.  The only change to products that inherit from
core.mk is removal of duplicated SELinux packages (which were present
in both core.mk and embedded.mk).

Change-Id: I6787bc42a6495c89ed63df7964512207a930d153