build: Change remaining xenbits.xen.org link to HTTPS

Obtaining code over an insecure transport is a terrible idea for
blatently obvious reasons.  Even for non-executable data, insecure
transports are considered deprecated.

This patch enforces the use of secure transports for all xenbits.xen.org
URLs.  All altered links have been tested and are known to work.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.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.xen.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.