Support UEFI PXE boot with DHCPv4

Support UEFI PXE boot with DHCPv4

pxelinux supports booting via BIOS, but to get EFI boot working via PXE
(especially when booting via Secure Boot even), then we need a proper
GRUB based boot toolchain.

By using binaries from grub-efi-amd64-signed + shim-signed packages and
integrating them in our configuration, we support BIOS boot via pxelinux
and EFI boot via GRUB at the same time.

FTR: to support grml-terminalserver as arch all package and not
*strictly* requiring the grub-efi-amd64-signed + shim-signed packages
(at least for now), we use a dependency chain as follows:

| grub-efi-amd64-signed | grub-efi-amd64-bin | pxelinux,
| shim-signed:amd64 | pxelinux,

This means that only pxelinux is strictly needed if you don't care about
PXE support, but need to ensure grub-efi-amd64-signed (or
grub-efi-amd64-bin as its fallback) and shim-signed are present on your
system if you want PXE boot support within grml-terminalserver.  This
change will be implemented in GRML_FULL class of grml-live for the
official Grml ISOs.

This work was funded by Grml-Forensic.