samba4: disable for avr32

The toolchain is not up to the task, fixes:
http://autobuild.buildroot.net/results/21f/21f5fd1fc8415abff1d1178843586956ef1fd1e3/


Also AVR32s are low on resources (RAM, no proper storage port, most of
them short of flash) to handle it.

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