Skip mount /oem on GSI

GSI self contains the full Android system part. Skip mounting
the oem partition on GSI, which is a part of system.

Bug: 158433698
Test: build aosp_arm64-userdebug.
Test: Check the file /system/system_ext/etc/init/config/skip_mount.cfg
Change-Id: I689e1cc8a68b980d68b1d6ef1d715203db078dc2
Merged-In: I689e1cc8a68b980d68b1d6ef1d715203db078dc2
(cherry picked from commit f8475cc49f78021e556a5803f688c096a90ca038)
diff --git a/target/product/gsi/gsi_skip_mount.cfg b/target/product/gsi/gsi_skip_mount.cfg
index 3f812cb..ad3c7d9 100644
--- a/target/product/gsi/gsi_skip_mount.cfg
+++ b/target/product/gsi/gsi_skip_mount.cfg
@@ -1,2 +1,3 @@
+/oem
 /product
 /system_ext