package/pkg-luarocks.mk: prepare infrastructure for host package support

package/pkg-luarocks.mk: prepare infrastructure for host package support

In preparation for the introduction of support for host Luarocks
packages, this commit reworks the luarocks-package infrastructure:

 - A number of variables that were defined in
   package/luarocks/luarocks-br-config.lua are now passed on the
   luarocks command line in $(2)_INSTALL_TARGET_CMDS in
   pkg-luarocks.mk.

 - The LUAROCKS_CFLAGS logic is moved from
   package/luarocks/luarocks.mk to package/pkg-luarocks.mk.

 - The LUAROCKS_RUN_ENV variable is no longer needed, as all details
   are now passed on the luarocks command line in pkg-luarocks.mk.

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