boot_serial: Allow direct image upload

The commit provides support for "image" parameter in mcumgr image
update frame, that is used to select image number to be updated;
for the purpose, the option MCUBOOT_SERIAL_DIRECT_IMAGE_UPLOAD
that enables the feature.
Platform that enables the feature needs, to provide an implementation
of the flash_area_id_from_direct_image function that will match
the image number to flash area to upload to.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>