Mistral: Support atmel keyboard emulator on Servo-V4

Added a support of atmel atmega32u4 keyboard emulator for Mistral.
This is required for FAFT test for switching developer and non
developer mode.

TEST=built hdctools and run the following command.
test_that --args='servo_board=mistral' -b mistral

Change-Id: Ic8ad12d3ebfa66cd638c5c1e89d770eb12ca4fe7
Reviewed-on: https://chromium-review.googlesource.com/1534041
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Ravutappa Bhosagi <rbhosagi@google.com>
Reviewed-by: Patrick Georgi <pgeorgi@chromium.org>
2 files changed