lib: add wchar strdup ...

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add beginning wchar support ...

EFI uses 16 bit character strings. Add beginning support for this.
Since barebox uses 8 bit strings internally we need conversion functions
to convert between 16 bit and 8 bit strings.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>