Use $(COMMAND) instead of `COMMAND`

The usage of $(COMMAND) shoud be preferred. Details can be
found at http://mywiki.wooledge.org/BashFAQ/082.

Also replaced two ` used in strings with '.

Change-Id: Ia729470c58da6fe2e505426bd5ce93214ec1f655
Merge "Use $(COMMAND) instead of `COMMAND`"