PS4 can include functions, so when running in the LOG_COLORS=True mode
provide a grey function line so that it's easier to visually
distinguish the content from the location.
Also make it so the main prompt chunks off all the common path, which
means we can printf to 40 characters and have a pretty reasonable and
readable PS4.
Change-Id: I4a10a49db97d413349bcfceeb8c4164936fbcc40