lua: Migrate to gentargets and improve it

Closes #803

Based on initial patch by rvpaasen@t3i.nl

[Peter: lua/luac needs liblua.so.* on target]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>