rpm: 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

 - pass --with-libintl-prefix only when a separate libintl library is
   available

 - dropping BR2_PACKAGE_GETTEXT selection

 - removing a musl-specific workaround. Indeed, when NLS is enabled,
   we now have the full-blown libintl, even with the musl C library

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