Improvements to the build script

[n1ai]

Improvements:

• Use the -e flag on the sh command so the script stops when it finds an error and reflects the error to the environment
• Make sure the user has provided an argument, and if not, issue an error message
• Make subdirectories relative to the current directory not absolute in /home/pi/sbitx
• Remove '-lopus' since we don't have a dependency on it
• Use blanks/tabs consistently