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

This reverts commit e1d7508444.

After spending ages sorting out Gitlab CI, it appears that OSSTest too has an
out-of-date Lets Encrypt cert.  Revert again in the short term while we fix
this up.

Signed-off-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 ?= https://xenbits.xen.org/xen-extfiles
+XEN_EXTFILES_URL ?= http://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.