lua: always use host-lua for host-luainterpreter

lua: always use host-lua for host-luainterpreter

Similar to how we do for openssl.

host-luainterpreter is only used by the luarocks infrastructure, and there
is afaik no specific reason why the host lua variant must match the target one.

Luajit only supports a limited number of architectures, so building it for
the host limits the architectures Buildroot can be used on (E.G. powerpc64
autobuilders).

To fix this, always use host-lua.  Slightly rework lua.mk to ensure
host-lua-5.1 is used when luajit is selected, and drop the logic for using
host-luajit.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>