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>
4a0f1f2817