luainterpreter: create virtual package

This patch introduces the luainterpreter virtual package, which
is provided either by 'lua' or by 'lua-jit'.

Packages that require a Lua interpreter can then depend on
BR2_PACKAGE_LUAINTERPRETER (in their Config.in) and luainterpreter
(in their .mk).

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[yann.morin.1998@free.fr: switch to package-defined providers,
                          apply Thomas' comments]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>