package/luabitop: depends on lua 5.1

package/luabitop: depends on lua 5.1

In preparation for the introduction of Lua 5.4, clarify the dependency
of luabitop: it only makes sense when used with Lua 5.1.

Also update the comment to no longer mention Lua 5.2, since we don't
support Lua 5.2 in Buildroot anymore.

Note that as explained in https://luajit.org/extensions.html, LuaJit
already implements luabitop functions, so luabitop is really for Lua
5.1 only, not for all Lua interpreters that implemented the 5.1 ABI.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>