MinPlatformPkg/Variable*Lib: Build VariableRead and VariableWrite libs

Add the VariableReadLib and VariableWriteLib instances to Components to
ensure build when building MinPlatformPkg.dsc.
Add a NULL library instance that provides the non-functional library
instance for VariableReadLib designed for all phase use.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Eric Dong <eric.dong@intel.com>
Signed-off-by: Isaac Oram <isaac.w.oram@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>