binutils: add avr32 special version

 * Add a new binutils version 2.18-avr32-1.0.1 to the Config.in

 * Use a different site to download avr32 binutils version in binutils.mk

 * Only apply patches from the patch directory if it exists in
   binutils.mk

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