multi-platform, new features

Tested, also tested upgrade (but requires changes in mcutool, which I will push there).

Upgrade from old to new version is more complicated:

1. First a specific version of application needs to be upgraded (190878c5). Afterwards the board needs to be restarted. This version (and newer) support upgrading bootloader.
2. Then bootloader and application needs to be upgraded to newest version.

I have prepared the compiled mcutool + required images + upgrading script at https://blackhole.sk/~kabel/tmp/mcutool.tar.gz.

To test upgrading on older boards (STM32, GD32, with or without user regulator, doesn't matter), just run:

cd /root
wget https://blackhole.sk/~kabel/tmp/mcutool.tar.gz
tar -xf mcutool.tar.gz
rm mcutool.tar.gz
cd mcutool
./upgrade.sh

# If the last command says
#   Please reboot and run again
# continue with

reboot

# wait until reboot

cd /root/mcutool
./upgrade.sh

# After this the firmware should be at newest version.

Note that RGB LED gamma correction is enabled by default with new firmware, and some old boards will need different gamma tables, because the LEDs are old. So we won't be doing automatic upgrade of MCU firmware for now.