Before this commit isolinux was used to boot an ISO and
thus forms a hard requirement. isolinux however only
exists for the x86 platform. This limitation did not
allow to create ISO images for other platforms. With this
commit a refactoring of the ISO boot setup is introduced.
* isolinux is only used if the bios firmware is requested
and the platform matches the x86 architecture. A runtime
check will check for this condition and exits early if
not applicable
* in case of the EFI firmware we already used grub in
EFI mode but still had isolinux in place for the legacy/CSM
boot. That part is now also replaced by a platform specific
grub eltorito image and grub's boot_hybrid.img for hybrid
boot. On platforms that do not provide those modules the
support for it will be skipped
With this change in place it's possible to control the ISO
boot layout through the firmware setup and all platform
specific modules are handled as such. Therefore we also
deleted the syslinux requirement. This Fixes#1092
dab7b8732a