build: Ensure /system unmounted at install start

Account for the possibility that a user has mounted system manually
before installing the FullOTA package.

Change-Id: I97d0540676a7ce98762545e76ecb1a7f6335d204
Signed-off-by: RITEFANG <1721985272@qq.com>

tools/releasetools/non_ab_ota.py

@@ -214,6 +214,7 @@ else if get_stage("%(bcb_dev)s") == "3/3" then
   # Dump fingerprints
   script.Print("Target: {}".format(target_info.fingerprint))
 
+  script.AppendExtra("ifelse(is_mounted(\"/system\"), unmount(\"/system\"));")
   device_specific.FullOTA_InstallBegin()
 
   CopyInstallTools(output_zip)