ShellPkg: make automatically created shells quit automatically

ShellPkg: make automatically created shells quit automatically

This creates a new parameter to this shell implementation “_exit” that causes the shell to automatically exit after running the initial specified command line.  This also automatically uses that command in the Execute() API.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: jaben carsey <jaben.carsey@intel.com>
Reviewed-by: Samer El-Haj-Mahmoud <elhaj@hp.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14444 6f19259b-4bc3-4df7-8a09-765794883524