samba: disable on AVR32

samba produces particularly large binaries, and the relocations needed
for it do not fit in the possible relocation mechanisms available on
AVR32. Since samba is very unlikely to be used on AVR32, let's just
disable it, as we've already done for samba4.

Fixes:

  http://autobuild.buildroot.net/results/0142b1d5b8c3f20f1bff76ece646d901407f7490/



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