Feature/quiet image building

[sfkleach]

Reduces the amount of noise during the build process by:

• Adding a -quiet flag to mkimage.p (and documenting it)
• Adding a lisp_debugging flag to the Common Lisp subsystem, defaulting to false, that reduces chatter
• Turning off chatter from the subsystems that respect the -d flag of the mk_cross script
• Making the Prolog and ML subsystems respect pop_debugging with regard to noisiness

Note: this builds on the popc -quiet flag branch