Better support for changing the CAN bit rate

[dc42]

We could support a small number of lower CAN bit rates (e.g. 500kbits/sec and 250kbits/sec) better if we changed the bootloader code and Duet 3 Expansion code to try listening for clock sync messages at various bit rates; and when one is received, if the bit rate doesn't match the bit rate configured in NVRAM, update the NVRAM. We would need a separate LED flash code for "no clock message received".