Remove all references to libintl

From now on, packages only need to select the BR2_PACKAGE_GETTEXT
option and depend on the 'gettext' package to get the necessary i18n
libraries installed on the target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: remove BR2_PACKAGE_LIBINTL]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
CC: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>