MdeModulePkg/SecurityPkg Variable: If a preexisting variable is rewritten with differ...

MdeModulePkg/SecurityPkg Variable: If a preexisting variable is rewritten with different attributes, SetVariable() shall not modify the variable and shall return EFI_INVALID_PARAMETER.

Two exceptions to this rule:
1. No access attributes specified
2. The only attribute differing is EFI_VARIABLE_APPEND_WRITE

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14552 6f19259b-4bc3-4df7-8a09-765794883524