net-tools: use the new gettext logic

This commit switches to use the new gettext logic, which involves:

 - using TARGET_NLS_DEPENDENCIES instead of hand-encoded dependencies
   on gettext/host-gettext

 - using TARGET_NLS_LIBS to force linking against libintl

 - dropping BR2_PACKAGE_GETTEXT selection

 - using BR2_SYSTEM_ENABLE_NLS instead of BR2_ENABLE_LOCALE to decide
   if NLS support should be enabled or not

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