bootstrap: avoid trying to run 'debug' command

If you invoke 'make debug', then bootstrap executes rebar:main/1 with
the extra argument "debug" and rebar treats that as a command. There is
no such command.

Without this patch you will see the following on bootstrap:
Command 'debug' not understood or not applicable
Merge pull request #200 from tuncer/fix-debug-bootstrap

bootstrap: avoid trying to run 'debug' command