ShellPkg/setvar: Support data format in Shell 2.2 spec

ShellPkg/setvar: Support data format in Shell 2.2 spec

Shell 2.2 spec defines =0x/=0X, =H/=h, =S, =L and =P for
hex number, hex array, ascii string, unicode string and
device path data.
The patch adds such support.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Jeff Fan <jeff.fan@intel.com>