WIP: Add support for STM32G0 with can split in separate file

[lichtfeind]

This is based on @ryedwards pull request #126 but tries to separate out the CAN relater changes into a separate file.

I am already on holiday so @marckleinebudde will continue to work in this.

This is developed on an NUCLEO-G0B1RE. And at the moment does not work yet. TX of CAN frames does not work and it only receives once.

TODO:

• Fix CAN
• rebase on multichannel #139
• Add some board definitions to support different pinmux configs between boards.

[marckleinebudde]

I've included the stm32g0b1 support in https://github.com/marckleinebudde/candleLight_fw/tree/multichannel, please close.