autoconf-archive: new package

This package contains a collection of freely re-usable autoconf
macros.

[Thomas:
 - change site to $(BR2_GNU_MIRROR), so that an official GNU site is
   used.
 - Change license to "GPLv3+ with exception", and add
   COPYING.EXCEPTION to the list of license files.]

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>