Introduce TARGET_BUILD_PDK for selecting build tweaks for the PDK

BUILD_PDK and some of the other infrastructure will go away once
we're done with the new new PDK work, but for now don't disturb
that stuff.

Change-Id: Ia39bf759ec03f203858ddcc4880af72bd132cb5a
diff --git a/core/tasks/apicheck.mk b/core/tasks/apicheck.mk
index b3d800b..8d9928e 100644
--- a/core/tasks/apicheck.mk
+++ b/core/tasks/apicheck.mk
@@ -18,7 +18,7 @@
 #
 
 # skip api check for TINY_ANDROID and PDK buid
-ifeq (,$(filter true, $(BUILD_TINY_ANDROID) $(BUILD_PDK)))
+ifeq (,$(filter true, $(BUILD_TINY_ANDROID) $(TARGET_BUILD_PDK)))
 
 .PHONY: checkapi