xen: arm: implement generic multiboot compatibility strings

xen: arm: implement generic multiboot compatibility strings

This causes Xen to accept the more generic names specified in
http://wiki.xen.org/wiki/Xen_ARM_with_Virtualization_Extensions/Multiboot as of
2014-06-06.

These names are more generic than those proposed by Andre in
http://thread.gmane.org/gmane.linux.linaro.announce.boot/326 and those
used in earlier drafts of the /Multiboot wiki page.

This will allow bootloaders to not special case Xen (or at least to reduce
the amount which is required).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>