tools: Add -Werror by default to all tools/

And provide an option to ./configure to disable it.

A follow-up patch will remove -Werror from every other Makefile in
tools/.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

config/Tools.mk.in

@@ -1,5 +1,6 @@
 -include $(XEN_ROOT)/config/Paths.mk
 
+CONFIG_WERROR       := @werror@
 CONFIG_RUMP         := @CONFIG_RUMP@
 ifeq ($(CONFIG_RUMP),y)
 XEN_OS              := NetBSDRump