build: auto-generate signed mcuboot binaries

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>