gcc: add avr32 special version

 * Add new gcc version 4.2.2-avr32-2.1.5 in Config.in

 * Select the Atmel mirror to download gcc with avr32 in their version
   name, in gcc-uclibc-*.mk

 * Do not apply patches if the patch directory is empty, in
   gcc-uclibc-*.mk

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