base_system.mk: Align formatting for updatable boot jar variable

Migrate PRODUCT_UPDATABLE_BOOT_MODULES &
PRODUCT_UPDATABLE_BOOT_LOCATIONS to a new PRODUCT_UPDATABLE_BOOT_JARS.
This new variable uses the same format as
PRODUCT_UPDATABLE_SYSTEM_SERVER_JARS, i.e <apex>:<jar> pair.

Test: Compiles & flashed device. Ensured that the bootclasspath & system_server class
paths remain the same.

Change-Id: I1cb26d5ec825cd1f5282a6e0255094ddf2fe046a
diff --git a/core/product.mk b/core/product.mk
index f451706..2276db2 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -366,8 +366,8 @@
 _product_single_value_vars += PRODUCT_BUILD_BOOT_IMAGE
 _product_single_value_vars += PRODUCT_BUILD_VBMETA_IMAGE
 
-_product_list_vars += PRODUCT_UPDATABLE_BOOT_MODULES
-_product_list_vars += PRODUCT_UPDATABLE_BOOT_LOCATIONS
+# List of boot jars delivered via apex
+_product_list_vars += PRODUCT_UPDATABLE_BOOT_JARS
 
 # Whether the product would like to check prebuilt ELF files.
 _product_single_value_vars += PRODUCT_CHECK_ELF_FILES