btrfs-progs: disable for avr32

Toolchain issues prevent it from being built, also there's no AVR32 with
proper storage ports for such a filesystem. Fixes:
http://autobuild.buildroot.net/results/06501fd32c30549810c12f0ad50f510c4b74ee63/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>