SecurityPkg/SecureBootConfigDxe: replace OpenFileByDevicePath() with UefiLib API

SecurityPkg/SecureBootConfigDxe: replace OpenFileByDevicePath() with UefiLib API

Replace the OpenFileByDevicePath() function with EfiOpenFileByDevicePath()
from UefiLib, correcting the following issues:

- imprecise comments on OpenFileByDevicePath(),
- code duplication between this module and other modules,
- local variable name "EfiSimpleFileSystemProtocol" starting with "Efi"
  prefix,
- bogus "FileHandle = NULL" assignments,
- leaking "Handle1" when the device path type/subtype check or the
  realignment-motivated AllocateCopyPool() fails in the loop,
- stale SHELL_FILE_HANDLE reference in a comment.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Roman Bacik <roman.bacik@broadcom.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1008
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>