linux-yocto: fix unapplied patch error message

linux-yocto: fix unapplied patch error message

When patches fail to apply, the status of all pending patches should
be exported to the logs and to the user. Currently, a missing export
of GUILT_BASE makes it look more like an internal error, than a 'normal'
patch failure:

    | [ERROR] unable to complete push
    | pending patches are:
    | Patches directory doesn't exist, try guilt-init

With this variable exported, we have this:

  | [INFO] validating against known patches  (qemux86-standard-meta)
  | error: patch failed: Makefile:2
  | error: Makefile: patch does not apply
  | To force apply this patch, use 'guilt push -f'
  | [ERROR] unable to complete push
  | pending patches are:
  | links/files/0002-makefile-patch.patch

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>