AVR32: fix libusb build

The libc of the avr32 toolchains seem to lack the timerfd-associated
functions.

Pass --disable-timerfd to libusb configure script when building for
avr32.

Fixes most of the recent avr32 breakages, such as
http://autobuild.buildroot.org/results/f418b6c0b026891c27467f61444b6eac8c105681
or
http://autobuild.buildroot.org/results/1a9e571998bc0a40822dca84267abbdd969bab5e



Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>