xen/boot: Print the build-id along with the changeset information

xen/boot: Print the build-id along with the changeset information

During initcalls is ok, but is a rather random place to find the build-id:

  (XEN) Parked 2 CPUs
  (XEN) build-id: 7ff05f78ebc8141000b9feee4370a408bd935dec
  (XEN) Running stub recovery selftests...

Logically, it is version information, so print with the changeset information
in console_init_preirq():

  (XEN) Xen version 4.13-unstable (andrewcoop@andrecoop) (gcc (Debian 4.9.2-10+deb8u2) 4.9.2) debug=y  Fri Apr 12 18:24:52 BST 2019
  (XEN) Latest ChangeSet: Fri Apr 5 14:39:42 2019 git:fc6c7ae-dirty
  (XEN) build-id: 7ff05f78ebc8141000b9feee4370a408bd935dec
  (XEN) PVH start info: (pa 0000ffc0)

Nothing has ever cared about xen_build_init()'s return value, so convert it to
void rather than include errno.h into the !BUILD_ID case of version.h

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>