Inconsitent use of BUILDDIR and BUILD_DIR

[c0mp3r]

Both variables mean the same but are used in different places. BUILD_DIR is the initialized one on the README while BUILDDIR isn't initialized anywhere. Should fix for consitency

[milimarg]

Also in hummingbird installation guide, there is $(BUILDDIR) written. But BUILDDIR is a variable so it should be $BUILDDIR.