CAN abort API for stm32

[alex31]

Hello,

I continue here in Small Change Requests a thread that has began here : viewtopic.php?f=35&t=176
I have wrote a couple of straightforward functions that does the job, only in low-level for now.
I have tested the behavior on a small can bus with three nodes + logic analyser, and it works in all the scenarios that i have tested : in fifo schedule mode, and priority based schedule mode.

the patch does not modify existing code, it just add two functions, one private, one public api. Patch is for 18.2.2, but since it just add functions, it should just work as is in trunk.

Alexandre