add hardware device descriptions to doc

most of the text is recycled and follows no particular format

devices/gl-mt300a/default.nix

@@ -12,10 +12,41 @@
   };
 
   description = ''
+    GL.Inet GL-MT300A
+    ********************
+
+    The GL-MT300A is based on a MT7620 chipset.
+
+    The GL-Inet pocket router range makes nice cheap hardware for
+    playing with Liminix or similar projects. The manufacturers seem
+    open to the DIY market, and the devices have a reasonable amount
+    of RAM and are much easier to get serial connections than many
+    COTS routers.
+
+    Wire up the serial connection: this probably involves opening
+    the box, locating the serial header pins (TX, RX and GND) and
+    connecting a USB TTL converter - e.g. a PL2303 based device - to
+    it. The defunct OpenWRT wiki has a guide with some pictures. (If
+    you don't have a USB TTL converter to hand, other options are
+    available. For example, use the GPIO pins on a Raspberry Pi.)
+
+    Run a terminal emulator such as Minicom on whatever is on the
+    other end of the link. I use 115200 8N1 and find it also helps
+    to set "Line tx delay" to 1ms, "backspace sends DEL" and
+    "lineWrap on".
+
+    When you turn the router on you should be greeted with some
+    messages from U-Boot and a little bit of ASCII art, followed by
+    the instruction to hit SPACE to stop autoboot. Do this and you
+    will get a gl-mt300a> prompt.
+
+    For flashing from uboot, the firmware partition is from
+    0xbc050000 to 0xbcfd0000.
+
     WiFi on this device is provided by the rt2800soc module. It
     expects firmware to be present in the "factory" MTD partition, so
     - assuming we want to use the wireless - we need to build MTD
-    support into the kernel even if we're using TFTP root
+    support into the kernel even if we're using TFTP root.
   '';
 
   module = { pkgs, config, lib, ...}: