Fixed problem that broke build in Kubuntu 14.04

This patch is part of the main buildroot repository but was never synced into
Atmel's buildroot build. The GCC documentation will no longer be generated on
the host machine.

From official buildroot repo:

Commit 62322ac, dated 2013-03-24 13:28:32 (GMT)

toolchain/gcc: disable makeinfo

Newer versions of texinfo (>=5) break the gcc makeinfo routine, so just
disable it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>