libuci: Lua binding needs mmu and version 5.1

The Lua binding option of libuci uses fork() so it needs the MMU.

Finally, libuci fails to build with Lua 5.2 because it uses functions
removed from this version. Fix it by activating the option only with
Lua 5.1.

[Thomas: use LIBUCI_ and not LIBUBOX_ otherwise the patch doesn't
work.]

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