xen: Allow EXPERT mode to be selected from the menuconfig directly

xen: Allow EXPERT mode to be selected from the menuconfig directly

EXPERT mode is currently used to gate any options that are in technical
preview or not security supported. At the moment, this is selected by
adding XEN_CONFIG_EXPERT=y on the make command line, or to the
(currently undocumented) top-level .config file.

This makes the option very unintuitive to use: If the user forgets to
add the option when (re)building or when using menuconfig, then
xen/.config will be silently rewritten, leading to behavior which is
very difficult to diagnose.  Adding XEN_CONFIG_EXPERT=y to the
top-level .config is not obvious behavior, particularly as the file is
undocumented.

A lot of the options behind EXPERT would benefit from being more
accessible so users can experiment with them and voice any concerns
before they are fully supported.

To make this option more discoverable and consistent to use, make it
possible to select it from the menuconfig.

This doesn't change the fact a Xen with EXPERT mode selected will not
be security supported.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>