iconv: disallow selection when BR2_ENABLE_LOCALE is enabled

A C library compiled with locale support already has an iconv()
implementation. In that case, libiconv is useless. Therefore, disallow
the selection of libiconv when the configuration says that the
toolchain has locale support.

This fixes build issues similar to the one we had with gettext:
libiconv headers are installed, but -liconv is not added because the
./configure script detects that the C library supports iconv.

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