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.