Eliminate std::wstring occurrences from uefi code

UEFI defines strings to use 16 bits wide code values. Use std::u16string
instead of wstring in C++ code for better portability.

Change-Id: Id8c2cdddb796a62a3aa14a5200038cd613ea1c17
Signed-off-by: Gabor Toth <gabor.toth2@arm.com>
Signed-off-by: Gyorgy Szing <gyorgy.szing@arm.com>
8 files changed