Pass TARGET_BUILD_PDK to soong

Also enable ALLOW_MISSING_DEPENDENCIES when TARGET_BUILD_PDK is set
so that soong modules can reference modules that are disabled in the
PDK.

Test: builds
Bug: 62086238
Change-Id: Ic43e843a717b802ace0cee568b9e7e561a6c0868
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 977b35c..e21083d 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -81,6 +81,7 @@
 	echo '    "Device_uses_hwc2": $(if $(filter true,$(TARGET_USES_HWC2)),true,false),'; \
 	echo '    "Override_rs_driver": "$(OVERRIDE_RS_DRIVER)",'; \
 	echo '    "Treble": $(if $(filter true,$(PRODUCT_FULL_TREBLE)),true,false),'; \
+	echo '    "Pdk": $(if $(filter true,$(TARGET_BUILD_PDK)),true,false),'; \
 	echo ''; \
 	echo '    "ArtUseReadBarrier": $(if $(filter false,$(PRODUCT_ART_USE_READ_BARRIER)),false,true),'; \
 	echo ''; \