Allow devices without AndroidBoard.mk

A long time ago, Android.mk in devices had the same semantics
as the current AndroidBoard.mk. This eventually caused problems
(as it didn't allow having a true Android.mk there) so we switched,
and left an error behind in case people were still using the old
semantics (i.e. having an Android.mk but no AndroidBoard.mk).

Fast forward a few years to the present: we're finding that we want
to get rid of AndroidBoard.mk, so there are devices that will have
Android.mk but not AndroidBoard.mk. As this is becoming a legitimate
case, the error for it is removed, and the include for AndroidBoard.mk
is made optional.

Change-Id: Ic683419d4aaab122259715e0112660d2a6359e91