Share system properties with all GSI project with treble_system.prop

Use TARGET_SYSTEM_PROP to assign a common system property file in each
GSI BoardConfig.mk

Bug: 62647757
Test: make and check the file system/build.prop in out folder
Change-Id: Ifb3e02356869f97573a3690a6d67c2b4a9471c46
Merged-In: Ifb3e02356869f97573a3690a6d67c2b4a9471c46
(cherry picked from commit 7e101699)