rpipico: Document picotool usage for filesystem

dc5145b has a comment indicating picotool should be used instead of
uf2conv but didn't document how to use picotool. This diff adds the
command that worked for me on my rp2350 based board (specifically an
adafruit_feather_rp2350).

Closes #1123

Kernel/platform/platform-rpipico/README.md

@@ -60,12 +60,13 @@ vi Makefile
 To build flash image use `make image -j`
 Or to build SD card image `make image-sd -j`
 
-You should now end up with `build/fuzix.uf2` and `filesystem.uf2`. The `uf2`
-files can be flashed onto the Pico in the usual way (i.e. connect it up as a
+You should now end up with `build/fuzix.uf2` and `filesystem.ftl`. The `uf2`
+file can be flashed onto the Pico in the usual way (i.e. connect it up as a
 mass storage device and copy the files on). Alternatively, you can use OpenOCD
 to load `build/fuzix.elf`. Once the board has rebooted it will appear as a USB
 serial device which you can connect to. Alternatively, connect a terminal to
-UART 0 on the Pico.
+UART 0 on the Pico. You can copy `filesystem.ftl` to the board using
+`picotool load filesystem.ftl -t bin -o 0x10018000`.
 
 If you want to use an SD card, note that only filesystems up to 32MB are
 supported.