Bug 831147: Add cmake to Mac bootstrap script.

The addition of apitrace introduces a new dependency on cmake.  Make sure
this is properly installed in the bootstrap script.  The wiki will need to
be updated separately.
Merge pull request #237 from wanderview/add-cmake-dep

Bug 831147: Add cmake to Mac bootstrap script.
Make get_about_memory.py work even if busybox is installed on the phone.
Merge pull request #238 from jlebar/get-about-memory-busybox

Make get_about_memory.py work even if busybox is installed on the phone. r=dhylands