Improve gettext integration

Having a BR2_NEEDS_GETTEXT option, as introduced by
54d64798 isn't sufficient to express
the different kind of dependencies on gettext.

This commit, based on an idea by Peter Korsgaard, introduces two
different options :

 * BR2_NEEDS_GETTEXT, which is true as soon as the toolchain doesn't
   provide gettext itself (i.e, when the toolchain is uClibc based, be
   it an internal or external toolchain)

 * BR2_NEEDS_GETTEXT_IF_LOCALE, which is true when the toolchain
   doesn't provide gettext *and* locale support has been enabled in
   Buildroot.

A following commit adds some documentation that details how these
configuration variables should be used by packages.

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