Add building rule.

Now we can use $(info ...) to check whether the rules
is correct or not, but still not run practically.
5 files changed
tree: cb71f146830c86c1552b62e887451c98a80171a9
  1. build/
  2. samples/
  3. sources/
  4. toolchains/
  5. .gitignore
  6. gdk-build