Split the code up and use BOOT_EFI and BOOT_BIOS to control which
goes where. We now support:
* BOOT_BIOS=1 BOOT_EFI=0 for BIOS boot only via an ElTorito boot
record (like debian-cd used to be before EFI support was added)
* BOOT_BIOS=1 BOOT_EFI=2 for standard BIOS boot as
primary ElTorito boot record and EFI as an alternate ElTorito boot
record (common case, just like we've been doing for amd64 in
debian-cd for a while)
* BOOT_BIOS=0 and BOOT_EFI=1 for *only* EFI boot as the primary
ElTorito boot record (new case, might be useful for some
Macs *maybe*)
Set BOOT_METHODS in the debian-cd environment to determine which of
these cases is desired in a given build.
Also removed the support for the "old" syslinux packaging layout, it's
not been around for a while now.
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