mkimg: Add support for offset if the source is an image

This allow us to create image with the following format:
mkimg -v -o sdcard -s gpt -p efi:=esp_aarch64.img:1M -p freebsd-ufs::1G

Which will add a efi partition at a 1M offset on the image with its content
coming from the esp_aarch64.img file.

MFC after: 3 days