libuci: correct lua path

LUAPATH is used as the path for installing lua binding.
As buildroot specifies the install prefix, STAGING_DIR gets
appended to TARGET_DIR which is incorrect path for installation,
so use relative path.

Reviewed-by: Abhimanyu Vishwakarma <Abhimanyu.Vishwakarma@imgtec.com>
Reviewed-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Neha Bairathi <Neha.Bairathi@imgtec.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>