scripts: west_commands: add sign command

This command is useful for signing binaries for loading by a
bootloader. At present, only MCUboot's "imgtool" is supported, but it
would be straightforward to add support for additional tools.

Using this command instead of "plain" imgtool avoids looking up any
numbers for the flash write block size, text section offset, or slot
size to get a signed binary. All users need to specify is the location
of the signing key.

This greatly improves usability for those unfamiliar with MCUboot, or
even experienced users who have to deal with multiple flash partition
layouts, boards, etc.

The command works by inspecting state in the Zephyr build system, some
of which is also provided by the runner package.

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