xen, tools: console.h shouldn't require string.h by default

xen, tools: console.h shouldn't require string.h by default

Unilaterally making string.h a prerequisite for console.h is going to
break build for a lot of consumers of console.h.

Define a macro for the new flex ring. Consumers which want to use it
should define the macro.

Partially revert af8d9356417cb617b635c5ace782388ebfe86e3a.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>