string: fix prototype of memdup()

string: fix prototype of memdup()

It doesn't make sense to restrict memdup() to only return char*
pointers, especially when it is already defined to accept void*. This
makes it uglier to use to e.g. duplicate a struct.

Make it return void*, just as kmemdup() does in the kernel (and which
our kmemdup() in fact also does).

While in here, make a small optimization: memcpy() is defined to
return the destination register, so we write this in a way that the
compiler may do a tail call.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>