Mark PRODUCT_SUPPORTS_xxx variables as single item instead of list

Evidenced by assignments across code base.

Test: treehugger
Bug: 116769560
Change-Id: I900217fc5745d3c29f844b968f9abc338b3a56ff
diff --git a/core/product.mk b/core/product.mk
index 2ab4b06..ce9bacf 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -224,10 +224,10 @@
 # instead of PRODUCT_BOOT_JARS, so that device-specific jars go after common jars.
 _product_list_vars += PRODUCT_BOOT_JARS_EXTRA
 
-_product_list_vars += PRODUCT_SUPPORTS_BOOT_SIGNER
-_product_list_vars += PRODUCT_SUPPORTS_VBOOT
-_product_list_vars += PRODUCT_SUPPORTS_VERITY
-_product_list_vars += PRODUCT_SUPPORTS_VERITY_FEC
+_product_single_value_vars += PRODUCT_SUPPORTS_BOOT_SIGNER
+_product_single_value_vars += PRODUCT_SUPPORTS_VBOOT
+_product_single_value_vars += PRODUCT_SUPPORTS_VERITY
+_product_single_value_vars += PRODUCT_SUPPORTS_VERITY_FEC
 _product_list_vars += PRODUCT_SYSTEM_SERVER_APPS
 _product_list_vars += PRODUCT_SYSTEM_SERVER_JARS
 # List of system_server jars delivered via apex. Format = <apex name>:<jar name>.