x86-64/EFI: allow chaining of config files

x86-64/EFI: allow chaining of config files

Namely when making use the CONFIG_XEN_COMPAT_* options in the legacy
Linux kernels, newer kernels may not be compatible with older
hypervisors, so trying to boot such a combination makes little sense.
Booting older kernels on newer hypervisors, however, has to always
work.

With the way xen.efi looks for its configuration file, allowing
individual configuration files to refer only to compatible kernels,
and referring from an older- to a newer-hypervisor one (the kernels
of which will, as said, necessarily be compatible with the older
hypervisor) allows to greatly reduce redundancy at least in
development environments where one frequently wants multiple
hypervisors and kernles to be installed in parallel.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>