toolchain: do not allow locale generation when locale support is not available

When the C library being used is uClibc, the locale support can be
disabled. In this case, it does not make sense to show the "Generate
locales" option.

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