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