wchar_t: wchar_t should go to stddef.h ...

according to C99 standard, the definition of wchar_t
should go stddef.h
this patch also remove wchar_t local definition and
use stddef.h instead.

Signed-off-by: Du Huanpeng <u74147@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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>