environment assertions at beginning of build.sh

[dargonforce]

Just a few issues I ran across when building an image from scratch. It would be best to check for them at the beginning of build.sh before a git-clone is issued or any other work is done

• check for required programs like hg
• check that you are running from the snakeware build system directory
  • consider maybe changing to the correct directory rather than just failing

[zachpmanson]

The dependencies also need to be added to the instructions if they aren't already there (mercurial, lsblk, git, kpartx)