Tools: added bootloader docs

Tools/AP_Bootloader/README.md

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+# ArduPilot Bootloader
+
+This is the bootloader used for STM32 boards for ArduPilot. To build
+the bootloader do this:
+
+```bash
+ ./waf configure --board BOARDNAME --bootloader
+ ./waf bootloader
+```
+
+the bootloader will be in build/BOARDNAME/bin. If you have the
+intelhex module installed it will build in both bin format and hex
+format. Both are usually uploaded with DFU. The elf file will be in
+build/BOARDNAME/AP_Bootloader for loading with gdb.
+
+The --bootloader option tells waf to get the hardware config from
+the hwdef-bl.dat file for the board. It will look in
+libraries/AP_HAL_CHibiOS/hwdef/BOARDNAME/hwdef-bl.dat
+
+The bootloader protocol is compatible with that used by the PX4
+project for boards like the Pixhawk. For compatibility purposes we
+maintain a list of board IDs here:
+
+  https://github.com/ArduPilot/Bootloader/blob/master/board_types.txt
+  
+the board IDs in that file match the APJ_BOARD_ID in the hwdef.dat and
+hwdef-bl.dat files
+
+The bootloader can load from USB or UARTs. The list of devices to load
+from is given in the UART_ORDER option in hwdef-bl.dat