lorax-embed-repo: Stop using extlinux by default

We're switching to GRUB2, which will support UEFI.
installer: Save all data from Lorax

Previously we were just doing boot.iso -> installer.iso, but in order
to support PXE we want to expose at least the extracted
kernel/vmlinuz/squashfs, and at that point we might as well just dump
it all.

If we need to subset it later, we can do that.