Fix Legacy GSI settings for BoardConfigGsiCommon.mk

Legacy GSI had been changed to include BoardConfigGsiCommon.mk
instead of treble_common*.mk. However, some default settings are
different than the original. This patch fixes
BOARD_BUILD_SYSTEM_ROOT_IMAGE in Legacy GSI makefiles.

The patch also removes redundant settings and fix some comments.

Bug: 122282239
Test: make
Change-Id: Ie4eb8cf410bfd2edc74648c0e5ee81028242a1b7
diff --git a/target/board/BoardConfigEmuCommon.mk b/target/board/BoardConfigEmuCommon.mk
index 55cd546..1e325b9 100644
--- a/target/board/BoardConfigEmuCommon.mk
+++ b/target/board/BoardConfigEmuCommon.mk
@@ -29,4 +29,4 @@
 DEVICE_MATRIX_FILE   := device/generic/goldfish/compatibility_matrix.xml
 
 BOARD_SEPOLICY_DIRS += device/generic/goldfish/sepolicy/common
-BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED := true
+