cmake: mcuboot: Add warning if missing Kconfig signing options

Adds a warning if neither Kconfig is enabled to generate an
unsigned image or a signing key are set, this is not an error but
warns the user in case they have forgot to set these modes or if
they have been set manually and have been cleared by a cmake
rebuild occurring (e.g. when bisecting).

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
1 file changed