ShellPkg: Update rm command to properly prepend file system identifiers to filenames.

This is required when recursively removing directories with files on file systems other than the current one.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <Jaben.carsey@intel.com>
Reviewed-by: Eugene Cohen <eugene@hp.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14441 6f19259b-4bc3-4df7-8a09-765794883524