retiring an old el6 host to do DHCP and TFTP from a shiny new el8 machine, I ran into a few issues when going through the PXE setup guide.

This MR has 4 commits with 4 separate changes I had to make to what was documented.
- for old clients, I find it useful to also have menu.c32 available over TFTP
- on both Fedora 31 and el8 the names of shim an and grub-efi now contain -x64
- the config file path for UEFI clients did not work for me
- since this is a worry I encoutre often (and also have), the default targets were changed so that a user is forced to actively select an install target

Shout if you prefer each of these four to be in a separate MR. I figured since the changes were small I'd just try to do clean commits but only one PR