fixed target "help" in Makefile

Without this commit, running `make help` produced the below output.

    $ make help
    Multi-OS Makefile, you are running on Linux
    Possible targets:
    + all - Make bootrom, armsrc and the OS-specific host directory
    + client - Make only the OS-specific host directory
    + flash-bootrom - Make bootrom and flash it
    /bin/sh: 1: Syntax error: "(" unexpected
    Makefile:20: recipe for target 'help' failed
    make: *** [help] Error 2

This issue was introduced in e335ca2846d57f660ab7f4c75b58226cca264e0e
six months ago, the above output was produced on Debian GNU/Linux, where
`/bin/sh` is usually a symbolic link to `/bin/dash`.