efi_loader: flesh out device-path to text

It needs to handle more device-path node types, and also multiple levels
of path hierarchy.  To simplify this, initially construct utf8 string to
a temporary buffer, and then allocate the real utf16 buffer that is
returned.  This should be mostly for debugging or at least not critical-
path so an extra copy won't hurt, and is saner than the alternative.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>