backuptool: Add an option to ensure running.

When updating via ROMManager, the presence of /tmp/.installscript
will keep backuptool from running. For people who don't like this
happen can `touch /system/etc/force_backuptool` to ensure the
running of backuptool.

Change-Id: I95b78ae569405b50d2cf95ddd05f0d554b32daad

prebuilt/common/bin/backuptool.sh

@@ -18,7 +18,7 @@ check_prereq() {
 }
 
 check_installscript() {
-   if [ -f "/tmp/.installscript" ];
+   if [ -f "/tmp/.installscript" -a ! -f "$S/etc/force_backuptool" ];
    then
       echo "/tmp/.installscript found. Skipping backuptool."
       PROCEED=0;
@@ -70,6 +70,7 @@ lib/libspeech.so
 lib/libvoicesearch.so
 etc/hosts
 etc/custom_backup_list.txt
+etc/force_backuptool
 EOF
 }