Can't create Mender images for Ubuntu Server x64

I think I found a different bug relating to the use of MENDER_GRUB_INITRD_IMAGETYPE in mender-convert-modify

https://tracker.mender.io/browse/MEN-3642

It looks like @drewmoseley found this as well, and there is a fix for this in https://github.com/mendersoftware/mender-convert/pull/211