edksetup.sh: rework argument parsing and update usage information

edksetup.sh: rework argument parsing and update usage information

Rework the argument parsing logic to a more extensible traditional
while/case setup to prepare for further additions.

While doing that, align the Usage printout to look a bit more like
the output of Edk2Setup.bat (and be more correct).

And also stop passing around command line options to the BuildEnv
script (which does not check them anyway).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>