Rename some Makefile targets. Closes #2274
Rename some Makefile targets. Closes #2274

kernel -> core
test_kernel -> test_stdlib
ggcampinho/elixir,beedub/elixir,antipax/elixir,antipax/elixir,gfvcastro/elixir,pedrosnk/elixir,joshprice/elixir,lexmag/elixir,pedrosnk/elixir,lexmag/elixir,kelvinst/elixir,ggcampinho/elixir,kimshrier/elixir,kimshrier/elixir,kelvinst/elixir,beedub/elixir,elixir-lang/elixir,gfvcastro/elixir,michalmuskala/elixir