fix(base_image_util.sh): Install glibc and gcc libs via emerge.
fix(build_image): Add --fast to control use of parallel_emerge

This makes it possible to toggle parallel_emerge just as other scripts
do. In other scripts update the help string to be more specific, the
--jobs option can be used to control parallelism.
fix(*): Change default for --jobs to $NUM_JOBS

This makes parallelism behavior between parallel_emerge and emerge
consistent. NUM_JOBS is defined as the number of CPUs by common.sh