This commit allows comments to reference files with paths that are relative
to the Zephyr workspace root. This is done by adding a new argument
'--workspace-dir' to the 'gen_edt.py' script, which is passed to the
'EDT' and 'DT' classes and used instead of the current working directory.
The workspace directory is set to WEST_TOPDIR if West is in use,
otherwise it is set to the parent directory of ZEPHYR_BASE so that
Zephyr files have a 'zephyr/' prefix.
Signed-off-by: Luca Burelli <l.burelli@arduino.cc>