Add at91sam3x8e CSP-over-CAN support

@jtrutna @jcappaert @bordicon 

This adds can_at91sam3x8e.c and updated wscript to allow for at91sam3x8e configuration option. The meat of the CSP-over-CAN effort is in the new file,  but the description of tests and effort can be found in the lemur-1-com pull request.

This is in conjunction with (and a submodule to) the larger lemur-1-com pull request.