package: rename DISABLE_NLS to NLS_OPTS

We are going to use DISABLE_NLS not only for disabling NLS support,
but also to explicitly enable it. Therefore, this preparatory commit
renames it to NLS_OPTS, which is consistent with other foo_OPTS
variables defined in package/Makefile.in.

While at it, we replace the := assignments by regular = assignments.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>