mtr: not available for avr32

The uclibc version condition is accurate but doesn't cover external
toolchains, so also exclude for avr32 too which only has 0.9.31 enabled.
Fixes:
http://autobuild.buildroot.net/results/4ee/4eec6e29d0f2a46bbbe3a748e85bd06c3601b7c1/



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