samd/boards/SAMD21_XPLAINED_PRO: Add specific deploy instructions.

Add instructions to install a bootloader to the board.  The SAMD21 XPLAINED
PRO board is shipped without a bootloader, which therefore has to be
installed once before it can be used with MicroPython.

Signed-off-by: robert-hh <robert@hammelrath.com>

ports/samd/boards/SAMD21_XPLAINED_PRO/board.json

@@ -1,6 +1,6 @@
 {
     "deploy": [
-        "../deploy.md"
+        "deploy_xplained_pro.md"
     ],
     "docs": "",
     "features": [

ports/samd/boards/SAMD21_XPLAINED_PRO/deploy_xplained_pro.md

@@ -0,0 +1,24 @@
+The SAMD21 Xplained Pro board is shipped without a bootloader. For use
+with MicroPyhton a suitable bootloader has be be installed first. The
+following procedure has been found to work and be simple:
+
+1. Get the bootloader from https://micropython.org/resources/firmware/bootloader-xplained-pro-v3.16.0-15-gaa52b22.hex.
+2. Connect your board to the debug port. A drive with the name XPLAINED
+shall appear.
+3. Copy the Intel hex file of the bootloader to that drive.
+4. Connect your board to the target port. A drive with the name SAMD21XPL should
+appear. If not, push reset twice. Then it should appear. If that does not
+happen, the bootloader was not properly installed or is not compatible.
+5. Copy the MicroPython firmware, a .uf2 file, to the SAMD21 drive. When the SAMD21
+drive disappears, MicroPython is installed.
+
+From now on only steps 4 and 5 are needed to update MicroPython. You can use the
+usual methods to invoke the bootloader, namely:
+
+- Push Reset twice.
+- Call machine.bootloader().
+- Use the touch 1200 procedure by switching the USB baud rate to 1200 baud and back.
+
+At the above link above there are as well .uf2 versions of the bootloader
+which one can install using steps 5. and 6. above once a .uf2 capable
+bootloader is installed.