west: commands: sign: look for imgtool.py in path

If 'imgtool' is not found in PATH look for 'imgtool.py' before bailing
out.

This allows adding the mcuboot/scripts directory to PATH and have
'west sign -t imgtool' automatically locate the 'imgtool.py' script.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>