update esp-idf to v3.3.6

[MaBecker]

EspruinoBuildTools have been updated, so this is good to go.

[gfwilliams]

Excellent - thanks! That's a lot cleaner than I was expecting!

[gfwilliams]

Just building this and for anyone else looking, you'll need:

rm -rf app
rm -rf esp-idf
rm -rf xtensa-esp32-elf
source scripts/provision.sh ESP32

If you were building before and you don't want to install from scratch

I wonder how many folks that'll catch out :)

Part of me thinks maybe we should have a file like version.txt in app and esp-idf that we can easily check in the provision script (or even Makefile)