There are currently two implementations of the utf16 string functions.
Somewhat confusingly, they've got different names.

Centralise the functions in efi.h.
Reviewed-by: NTom Gundersen <teg@jklm.no>
Tested-by: NTom Gundersen <teg@jklm.no>
Reviewed-by: NMike Waychison <mikew@google.com>
Signed-off-by: NMatt Fleming <matt.fleming@intel.com>