bitbake: bin/bitbake: Improve --help text

bitbake: bin/bitbake: Improve --help text

The --help text was rather inconsistent in style and plain incorrect in places,
using confusing terminology in others. I guess most people know what the options
do and don't read this but its confusing to new users.

This updates it to use the terms recipe and task consistently, remove
the references to stage, bbread and generally try and make the output
more useful.

[YOCTO #4856]

(Bitbake rev: 516311946c7bd14c84947dc44c3bb0563e5a9667)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>