Depend on either syslinux or grub2-common + grub-pc-bin

grml2usb depends on grub-pc(-bin) files and grub2-common's grub-install,
but if someone has grub-efi-amd64 installed, then the grub-pc(-bin)
files are missing. The grub-efi-amd64 package shouldn't matter at all,
since we explicitly run grub with `--target=i386-pc`. Adjust depends
accordingly.

Closes: #796801
Thanks: Albert Dengg for bugreport and suggested solution + Axel Beckert for assistance in getting a proper dependency resolution