Some commands may cause current directory or even current mapping
become invalid. For example, "MAP" after unplugging the USB thumb
key may cause current mapping "FS0:" disappear.
This patch updates the CWD and current mapping when commands return.
It also causes the command prompt change to default "Shell>".
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Huajing Li <huajing.li@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by Jaben Carsey <jaben.carsey@intel.com>
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