Fix signing on some GSI targets

GSI targets require enabling chain partition signing, which put the
GSI signing in system.ing directly instead of in vbmeta.img.

Some GSI targets inherit from BoardConfigGsiCommon.mk only, ex.
gsi_arm64-user, do not enable chain partition signing. The patch
add these settings back.

On the other hand, the emulator makefiles still requires the
settings at this time. Hence, we keep these settings in
BoardConfigEmuCommon.mk.

Bug: 150179516
Test: make gsi_arm64-user
Test: make aosp_arm64-user
Change-Id: Id188e4c6a31ce0cf9b134ebae36cbd5bfae19942
Merged-In: Id188e4c6a31ce0cf9b134ebae36cbd5bfae19942
(cherry picked from commit f77e364d)