Make `init_boot.img` available if it exists

[petefoth]

The Flashing additional partitions section of the LineageOS installation instructions for Moto X40 rtwo require an init_boot.img file as well as boot, dtbo&vendor_boot` images.This is a relatively new requirement - see this commit in the LineageOS Wiki repo

We need to add it to the list of images that we handle in build.sh so that the file gets uploaded to the download server.

Thanks to XDA From member Big Dumb Oblect for the report

[petefoth]

Fix ready for testing in pf-add-boot_init-image branch

[petefoth]

Test by

• making a build for rtwo using the pf-add-boot_init-image docker image:
• check that boot_init.img is uploaded to https://download.lineage.microg.org/rtwo/

Do this after the August build run is complete