package/pkg-luarocks.mk: use custom luarocks config file

package/pkg-luarocks.mk: use custom luarocks config file

In preparation for the addition of the support for host-luarocks
packages, this commit changes the luarocks logic to use a custom
configuration file in $(HOST_DIR)/etc/luarocks/config.lua instead of
the default
$(HOST_DIR)/etc/luarocks/config-$(LUAINTERPRETER_ABIVER).lua.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[Thomas: extracted from a larger patch from François]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>