west: sign.py: usability fixes

Fix some usability issues with this command.

- add help making it clear that either imgtool must be installed, or
  the path to imgtool.py must be provided using --tool-path.

- in case people don't read that, print a helpful message if imgtool
  is not installed and --tool-path is not provided.

- in case the build directory is not properly set up for an MCUboot
  chain-loaded image, make the BuildConfiguration inspection more
  robust, printing out errors using check_force() when values are
  missing.

- mark the --tool option required to print usage and avoid a
  RuntimeError if it is not provided.

- make sure we search for the default build directory before checking
  for its existence, in case it was not provided

Signed-off-by: Marti Bolivar <marti@foundries.io>