luarocks: fix target-finalize hook processing

The LUAROCKS_TARGET_FINALIZE_HOOKS is not running, so detritus is being left
in /usr/lib/luarocks.  This is because host-luarocks is built by being a
dependency in the luarocks package infrastructure, not by being selected by
kconfig symbol.  This means that the $(PKG)_KCONFIG_VAR in pkg-generic.mk is
not met, and (HOST_)LUAROCKS_TARGET_FINALIZE_HOOKS is not added to the
global TARGET_FINALIZE_HOOKS.

This mod fixes this issue by adding the host-luarocks hook directly
to TARGET_FINALIZE_HOOKS when either lua or luajit is enabled.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>