Remove /system -> /system_root/system symlink in ZIP-install

when unmounting system /system_root/system is symlinked to
/system on devices built with BOARD_BUILD_SYSTEM_ROOT_IMAGE

Some ZIPs don't play well with a symlink when trying to mount to
/system.

Change-Id: Ifb0a105ddc0337353a7e1ac234c9f1e5b387e992

twinstall.cpp

@@ -419,6 +419,8 @@ int TWinstall_zip(const char* path, int* wipe_cache) {
 			gui_err("unmount_system_err=Failed unmounting System");
 			return -1;
 		}
+		unlink("/system");
+		mkdir("/system", 0755);
 	}
 
 	time_t start, stop;