elfutils: produce correct libelf.pc file when building with full nls

When building with full lagnuage support, libelf.so will depend on and
link with libintl.so so we need to change the pkg-config template to
reflect this library dependency.

Also change the Makefile to only pass --disable-nls to configure when
the full nls support is actually disabled in the buildroot config.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>