Add umount call to init to unmount a filesystem

Bug: 29278988
Change-Id: Ia14ab1bcad9513688df97f7b4d9162a9ab282480

init/builtins.cpp

@@ -345,6 +345,11 @@ static int do_mkdir(const std::vector<std::string>& args) {
     return 0;
 }
 
+/* umount <path> */
+static int do_umount(const std::vector<std::string>& args) {
+  return umount(args[1].c_str());
+}
+
 static struct {
     const char *name;
     unsigned flag;
@@ -958,6 +963,7 @@ BuiltinFunctionMap::Map& BuiltinFunctionMap::map() const {
         {"mkdir",                   {1,     4,    do_mkdir}},
         {"mount_all",               {1,     kMax, do_mount_all}},
         {"mount",                   {3,     kMax, do_mount}},
+        {"umount",                  {1,     1,    do_umount}},
         {"powerctl",                {1,     1,    do_powerctl}},
         {"restart",                 {1,     1,    do_restart}},
         {"restorecon",              {1,     kMax, do_restorecon}},

init/readme.txt

@@ -355,6 +355,9 @@ trigger <event>
    Trigger an event.  Used to queue an action from another
    action.
 
+umount <path>
+   Unmount the filesystem mounted at that path.
+
 verity_load_state
    Internal implementation detail used to load dm-verity state.