Allow product-specific release configs.

Bug: 343794062
Test: manual
Change-Id: Ieb47ce685c7f29c0e8afe8897f86ffebb6318396
diff --git a/core/release_config.mk b/core/release_config.mk
index 4fdd279..2898868 100644
--- a/core/release_config.mk
+++ b/core/release_config.mk
@@ -130,6 +130,7 @@
         # Disable the build flag in release-config.
         _args += --guard=false
     endif
+    _args += --allow-missing=true
     _flags_dir:=$(OUT_DIR)/soong/release-config
     _flags_file:=$(_flags_dir)/release_config-$(TARGET_PRODUCT)-$(TARGET_RELEASE).vars
     # release-config generates $(_flags_varmk)