change assorted xenbits.xen.org links to HTTPS

Also switch to xenbits.xenproject.org at this occasion.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

Config.mk

@@ -191,7 +191,7 @@ APPEND_CFLAGS += $(foreach i, $(APPEND_INCLUDES), -I$(i))
 EMBEDDED_EXTRA_CFLAGS := -fno-pie -fno-stack-protector -fno-stack-protector-all
 EMBEDDED_EXTRA_CFLAGS += -fno-exceptions -fno-asynchronous-unwind-tables
 
-XEN_EXTFILES_URL ?= http://xenbits.xen.org/xen-extfiles
+XEN_EXTFILES_URL ?= https://xenbits.xenproject.org/xen-extfiles
 # All the files at that location were downloaded from elsewhere on
 # the internet.  The original download URL is preserved as a comment
 # near the place in the Xen Makefiles where the file is used.