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