Cleanup bash code style

There was a lot to clean up.  I ended up using vim search and replace on selection to help me out with replacing all of the `$var` with `${var}`.  I used the following expression to help me with substitution.  It is kind of a garbled mess to look at but it has been battle tested throughout all of the scripts one by one.  It does not play nice with `sed` unfortunately due to quoting issues.

    :'<,'>s#$\([^0-9(\{]\{1\}[^/{} '"()\\;.|`+*-]\+\)#${\1}#g

I also quoted where I knew would make sense like when using `cd` and other commands or surrounding command substitution.  Some of the places I didn't quote but should probably be quoted is multiline command substitution where `python` is executed with a "here doc".  The multiline command substitutions can be found in the following files:

* `profiles/devices/acer-c720/scripts/reinstall-vmlinuz.sh`
* `profiles/devices/acer-c720/scripts/trusty-patch.sh`

I didn't change them because I didn't know what the behavior would be and don't really have time to test it at the moment.

Also a BIG DISCLAIMER before you merge these patches.  I don't have a Chromebook at all so my changes are largely untested.  The only way I could really test was just for syntax from the project root with the following command:

    find . -type f -name '*.sh' -o -type f -name '*.profile' -print0 | xargs -0 -I '{}' bash -n '{}'

These changes partially come from following the [Google styleguide for shell scripts](http://google-styleguide.googlecode.com/svn/trunk/shell.xml).  Please do me a favor by testing my changes since I'm not able.  Thanks.