system: introduce BR2_SYSTEM_ENABLE_NLS

Until now, the option BR2_ENABLE_LOCALE was more-or-less controlling
whether NLS support was enabled in packages. More precisely, if
BR2_ENABLE_LOCALE=y, we were not doing anything (so some packages
could have NLS support enabled, some not). And only when
BR2_ENABLE_LOCALE was disabled we were explicitly passing
--disable-nls to packages.

This doesn't make much sense, and there is no reason to tie NLS
support to locale support. You may want locale support, but not
necessarily NLS support. Therefore, this commit introduces
BR2_SYSTEM_ENABLE_NLS, which allows to enable/disable NLS support
globally. When this option is enabled, we pass --enable-nls to
packages, otherwise we pass --disable-nls.

In addition, when this option is enabled and the C library doesn't
provide a full-blown implementation of gettext, we select the gettext
package, which will provide the full blown implementation.

It is worth mentioning that this commit has a visible impact for users:

 - Prior to this commit, as soon as BR2_ENABLE_LOCALE=y, packages
   *could* provide NLS support. It was up to each package to decide
   whether they wanted to provide NLS support or not (we were not
   passing --enable-nls nor --disable-nls).

 - After this commit, it's BR2_SYSTEM_ENABLE_NLS that controls whether
   NLS is enabled or disabled, and this option is disabled by default.

Bottom line: with the default of BR2_SYSTEM_ENABLE_NLS disabled, some
packages may lose NLS support that they used to provide. But we
believe it's a reasonable default behavior for Buildroot, where
generally NLS support is not necessary.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>