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
diff --git a/target/board/Android.mk b/target/board/Android.mk
index ac8cb44..82dee3c 100644
--- a/target/board/Android.mk
+++ b/target/board/Android.mk
@@ -23,16 +23,7 @@
 # Use the add-radio-file function to add values to this variable.
 INSTALLED_RADIOIMAGE_TARGET :=
 
-ifeq (,$(wildcard $(TARGET_DEVICE_DIR)/AndroidBoard.mk))
-  ifeq (,$(wildcard $(TARGET_DEVICE_DIR)/Android.mk))
-    $(error Missing "$(TARGET_DEVICE_DIR)/AndroidBoard.mk")
-  else
-    # TODO: Remove this check after people have had a chance to switch,
-    # after April 2009.
-    $(error Please rename "$(TARGET_DEVICE_DIR)/Android.mk" to "$(TARGET_DEVICE_DIR)/AndroidBoard.mk")
-  endif
-endif
-include $(TARGET_DEVICE_DIR)/AndroidBoard.mk
+-include $(TARGET_DEVICE_DIR)/AndroidBoard.mk
 
 # Generate a file that contains various information about the
 # device we're building for.  This file is typically packaged up