Move utf16 functions to kernel core and rename

We want to be able to use the utf16 functions that are currently present
in the EFI variables code in platform-specific code as well. Move them to
the kernel core, and in the process rename them to accurately describe what
they do - they don't handle UTF16, only UCS2.

Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>