Updating level 1 commands.

Updating level 1 commands.

exit - correctly convert the parameter to the return code.
for  - update string-to-number conversion.
if   - use shell string-to-number conversion and add comments.
stall- add command

main lib files:
add comments, clarify user visible strings, add stall.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@11420 6f19259b-4bc3-4df7-8a09-765794883524