Where to put custom make targets?

I just assumed I could add custom make targets to the Makefile, and I can! but `concrete update` really wants to use it's Makefile template. I can respect that.

I was thinking we could have the Makefile try and `include custom.mk` but be totally ok if that file doesn't exist. This way you don't have to worry about merging your custom make targets back in from `Makefile.0`.

I'll do the work, just let me know if you're all cool with the solution first.

cc @seth @jamesc