robot tests: Rework the scripts to group common bits

Prelude to more refactoring and getting this whole
set of robot tests running on FPGA in CI.

  - Adjust FPGA header file to include quiesce delay.
  - Adjust fpga_test.sh to copy the whole tests dir.
  - Put variables into variables/ as python modules
    and depend on environment variables for ROOTDIR and
    PLATFORM.
  - Move common keywords into resources/common.resource.
  - Adjust variable names like UART5 to more sensible
    names.
  - Adjust ML test to use a variable for the model name.
  - Remove tarball reconstruction script from
    shodan_boot.robot since we don't actually need that
    for testing in renode.

Change-Id: I53aa03a353d83b6f51ff3c615df5015e47fd18f6