Disable automatic update checks if booting from a removable device.

utils.h

Index: utils.h
diff --git a/utils.h b/utils.h
index f7f734ed322bf38ae2a1fd088e8661b170999b5b..6ce4cf36ad20ed219ddc8d681af902af63b58024 100644
--- a/utils.h
+++ b/utils.h
@@ -84,13 +84,24 @@ bool MakeTempDirectory(const std::string& dirname_template,
 bool RecursiveUnlinkDir(const std::string& path);
 
 // Returns the root device for a partition. For example,
-// RootDevice("/dev/sda3") returns "/dev/sda".
+// RootDevice("/dev/sda3") returns "/dev/sda". Returns an empty string
+// if the input device is not of the "/dev/xyz" form.
 std::string RootDevice(const std::string& partition_device);
 
 // Returns the partition number, as a string, of partition_device. For example,
-// PartitionNumber("/dev/sda3") return "3".
+// PartitionNumber("/dev/sda3") returns "3".
 std::string PartitionNumber(const std::string& partition_device);
 
+// Returns the sysfs block device for a root block device. For
+// example, SysfsBlockDevice("/dev/sda") returns
+// "/sys/block/sda". Returns an empty string if the input device is
+// not of the "/dev/xyz" form.
+std::string SysfsBlockDevice(const std::string& device);
+
+// Returns true if the root |device| (e.g., "/dev/sdb") is known to be
+// removable, false otherwise.
+bool IsRemovableDevice(const std::string& device);
+
 // Synchronously mount or unmount a filesystem. Return true on success.
 // Mounts as ext3 with default options.
 bool MountFilesystem(const std::string& device, const std::string& mountpoint,