Skip iso/pxe/syslinux on arm64

iso/pxe/syslinux are x86-specific. The presence of the files and
boot code is entirely useless on arm64.

While at it, update netboot code for arm64-specific shim and grub
filenames.

etc/grml/fai/config/package_config/GRMLBASE

@@ -29,7 +29,6 @@ grml-udev-config
 hdparm
 hwinfo
 initramfs-tools
-isolinux
 kbd
 less
 live-boot-grml live-boot-grml-doc
@@ -40,7 +39,6 @@ openssh-client
 openssh-server
 pciutils
 physlock
-pxelinux
 resolvconf
 rsync
 rsyslog
@@ -81,13 +79,17 @@ PACKAGES install I386
 grub-pc
 grub-efi-amd64-bin
 grub-efi-ia32-bin
+isolinux
+pxelinux
 syslinux syslinux-common syslinux-utils
 grml2usb
 
 PACKAGES install AMD64
 grub-pc
 grub-efi-amd64-bin
 grub-efi-ia32-bin
+isolinux
+pxelinux
 syslinux syslinux-common syslinux-utils
 grml2usb