[build] Allow for building EFI ISOs without syslinux

[build] Allow for building EFI ISOs without syslinux

The isohybrid tool (that creates a partition table to describe the ESP
embedded within the ISO image) relies upon the existence of the
x86-only isolinux.bin file.  This file may not be available on non-x86
build hosts.

We already allow for the possibility that isohybrid may not be present
on the build host, in which case we fall back to creating a non-hybrid
ISO image.  Make a similar allowance for the possibility that the
isolinux.bin file may not be present: require its existence only if we
are attempting to build a BIOS-bootable image, or if we will be
attempting to use isohybrid to create a hybrid ISO image.

Reported-by: Xiaotian Wu <wuxiaotian@loongson.cn>
Signed-off-by: Michael Brown <mcb30@ipxe.org>