Fixing the template is pretty easy. I got it to look right with:

<% kernel, initrd = @host.operatingsystem.boot_files_uri(@host.medium,@host.architecture) -%>
wget -O "$KERNEL" "<%= kernel %>" 
wget -O "$INITRD" "<%= initrd %>" 

However, grubby isn't available in Debian. It may or may not be available in Ubuntu; I don't run it so I don't know.

Hm, that's right, no grubby on Debian and also on Ubuntu... but the template approach seems to be much easier than modifying the actual foreman code, so I'll stick with that. ;)

The opposite is also true; @host.operatingsystem.boot_files_uri(@host.medium,@host.architecture) won't work on RH systems, which means that for example for a Red Hat PXELinux ONLY

<%
kernel = @host.url_for_boot(:kernel)
initrd = @host.url_for_boot(:initrd)
-%>

KERNEL <%= kernel %>
INITRD  <%= initrd %>

works

While for a Debian ONLY

<% kernel, initrd = @host.operatingsystem.boot_files_uri(@host.medium,@host.architecture) -%>

KERNEL <%= kernel %>
INITRD  <%= initrd %>

works.

Kinda confusing if you ask me ;)

That can be handled via ERB in templates. In fact that's necessary as ks=... won't work for Debian or SUSE. The problem I'm having is, that accessing boot_files_uri() requires to have safemode disabled, so that's not a way to go. I lean towards closing that issue with the lame excuse of grubby not being packaged in Debian/Ubuntu...