Finalised SAMD port

[algernon]

Turns out that my previous attempts at a SAMD port weren't enough, and there are a number of things left to fix up. This patchset "fixes" the host wakeup code (previously it compiled, but failed to link), and updates a few places that need special handling on SAMD.

The host wakeup code compiles and links, but doesn't work yet. I left the attempt in regardless, because it's a pointer to where we should look when debugging it further.

Tested the updates on both AVR and SAMD, and everything appears to function correctly, apart from the wakeup on SAMD.

[algernon]

Update:

• The SendReport stuff is not needed, PackMessages is enough.
• Things work on both AVR and SAMD.

I'll clean things up and test wakeup, then push an update.

[algernon]

Wakeup doesn't seem to work on SAMD, will have to re-test with AVR too, just in case.