Add new Kconfig options which depend on CONFIG_BOOTLOADER_MCUBOOT:
- CONFIG_MCUBOOT_SIGNATURE_KEY_FILE: the path to the key pair which
should be used to sign the image, in PEM format. This is sent to
imgtool via 'west sign' when set to produce zephyr.signed.bin and
zephyr.signed.hex files as needed.
- CONFIG_MCUBOOT_EXTRA_IMGTOOL_ARGS: additional arguments to pass to
imgtool.
- CONFIG_MCUBOOT_GENERATE_CONFIRMED_IMAGE: also generate
zephyr.signed.confirmed.{bin,hex}
Add build system support for these options.
This makes a separate 'west sign' step unnecessary when using MCUboot,
if the application image is properly configured with the location of
the key file.
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>