jztool: New utility for installing a bootloader on FiiO M3K

At present, this is just a command line tool for Linux only.

Integrating this with the Rockbox utility and porting to other
platforms should be straightforward; the README contains more
information.

Change-Id: Ie66fc837a02ab13c878925360cabc9805597548a

rbutil/jztool/README.md

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+# jztool -- Ingenic device utility & bootloader installer
+
+The `jztool` utility can install, backup, and restore the bootloader on
+Rockbox players based on a supported Ingenic SoC.
+
+## FiiO M3K
+
+To use `jztool` on the FiiO M3K you have to connect the player to your
+computer in USB boot mode.
+
+The easiest way to do this is by plugging in the microUSB cable to the M3K
+and holding the volume down button while plugging the USB into your computer.
+If you entered USB boot mode, the button light will turn on but the LCD will
+turn off.
+
+To install or update the Rockbox bootloader on the M3K, use the command
+`jztool fiiom3k install`. It is recommended that you take a backup of your
+current bootloader so you can restore it in case of any problems.
+
+After any operation finishes, you will have to force a power off of the M3K
+by holding down the power button for at least 10 seconds. This must be done
+whether the operation succeeds or fails. Just don't power off or unplug the
+device in the middle of an operation -- that might make bad things happen.
+
+See `jztool --help` for info.
+
+## TODO list
+
+### Add better documentation and logging
+
+There's only a bare minimum of documentation, and logging is sparse, not
+really enough to debug problems.
+
+Some of the error messages could be friendlier too.
+
+### Integration with the Rockbox utility
+
+Adding support to the Rockbox utility should be mostly boilerplate since the
+jztool library wraps all the troublesome details.
+
+Getting appropriate privileges to access the USB device is the main issue.
+Preferably, the Rockbox utility should not run as root/admin/etc.
+
+- Windows: not sure
+- Linux: needs udev rules or root privileges
+- Mac: apparently does not need privileges
+
+### Porting to Windows
+
+Windows wants to see a driver installed before we can access the USB device,
+the easiest way to do this is by having the user run Zadig, a 3rd party app
+which can install the WinUSB driver. WinUSB itself is from Microsoft and
+bundled with Windows.
+
+Zadig's homepage: https://zadig.akeo.ie/
+
+### Porting to Mac
+
+According to the libusb wiki, libusb works on Mac without any special setup or
+privileges, presumably porting there is easy.