ectool: add support for the MEC LPC protocol

[DHowett]

The MEC LPC protocol is supported in Linux' cros_ec_lpcs driver, but not in ectool's "local" implementation. However, cros_ec_lpcs will not load on the Framework Laptop due to an I/O port conflict^1. Therefore, ectool's local implementation is required to support the MEC protocol on kernel versions that do not contain a fix for the I/O conflict.

I've not taken care to deduplicate the protocol code with comm-lpc, as there is little value in sharing due to the different order in which command data are checksummed.