Add support for SerialPortLib that maps into POSIX StdIn and StdOut. Add a device...

Add support for SerialPortLib that maps into POSIX StdIn and StdOut. Add a device path text to lib as a holding point waiting on getting it reviewed for other packages. Some minor fixes. Also map the FV as writable, so the Variable store becomes writable.

I plan to try and make only the Variable store and logs writable, and make the executable/compressed FV read only in a future checkin.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@11760 6f19259b-4bc3-4df7-8a09-765794883524