xen/xsm: Don't use multiboot by default to initialize XSM

xen/xsm: Don't use multiboot by default to initialize XSM

Multiboot protocol is only used by x86. When XSM support for ARM will be
introduced, another way will be used to retrieve the policy blob.

Introduce CONFIG_MULTIBOOT to compile XSM specific multiboot code and
rename xsm_init into xsm_multiboot_init to avoid confusion.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Keir Fraser <keir@xen.org>