MinPlatformPkg: Add PcdDefaultTerminalType support to SerialPortTerminalLib

 - Sets the terminal type GUID for ConIn, ConOut, and ConErr to the terminal
   type indicated by PcdDefaultTerminalType.
 - Some improvements to the comments in SerialPortTerminalLib

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Sai Chaganty <rangasai.v.chaganty@intel.com>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Benjamin Doron <benjamin.doron00@gmail.com>
Cc: Michael Kubacki <michael.kubacki@microsoft.com>
Cc: Jeremy Soller <jeremy@system76.com>
Signed-off-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>