Generate additional boot image for eMMC boot hwpart

Feature: https://source.puri.sm/Librem5/use-cases/issues/60

This merge request adds support for generating an additional boot image alongside the main one.

This is required to be able to flash U-boot and the M4 firmware into one of the eMMC boot hwparts.

To test the changes in the build-image script and the Makefile, add this for devkit in conf/board:

want_boot_image=1

To also test the Jenkinsfile, add 'devkit' in the separate_boot_image list.