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/main.mk b/core/main.mk
index ab9d52d..a038d5d 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -218,7 +218,8 @@
 endif
 ifneq ($(PDK_BUILD_TYPE),)
   $(info PDK build type $(PDK_BUILD_TYPE))
-  BUILD_PDK:= true
+  BUILD_PDK:=true
+  TARGET_BUILD_PDK:=true
   include pdk/build/pdk.mk
   # force droid target
   MAKECMDGOALS:= $(subst $(PDK_BUILD_TYPE),droid,$(MAKECMDGOALS))
@@ -729,7 +730,7 @@
 ifeq ($(BUILD_TINY_ANDROID), true)
 INSTALLED_RECOVERYIMAGE_TARGET :=
 endif
-ifneq ($(BUILD_PDK),)
+ifneq ($(TARGET_BUILD_PDK),)
 INSTALLED_RECOVERYIMAGE_TARGET :=
 endif