toolchain: purge locales by default

Our current default is to keep all locales installed in
/usr/share/locale/. However, in practice, those locales take up a
significant amount of space, and most users do not need
locales. Therefore, it makes more sense to default to purging locales,
in order to keep only a few useful ones rather than keeping them all.

It helps in providing a small filesystem size by default, and still
allows advanced users who really need locales to tune their
configuration.

As an example, a very basic system with just util-linux enabled (not
even Busybox) weights 11 MB, including 6.4 MB of locales. With this new
default, the generated system is only 4.2 MB.

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