uclibc: add avr32 special version

 * Add a new 0.9.30-avr32-2.1.5 version of uClibc in Config.in

 * Add a patch for 0.9.20-avr32-2.1.5

 * Select a different UCLIBC_SITE for avr32 versions in uclibc.mk

 * Add the LINKRELAX=y configuration option to the uClibc .config file
   in uclibc.mk

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