lib: add u16_strcpy/strdup functions

Add u16_strcpy() and u16_strdup(). The latter function will be
used later in implementing efi HII database protocol.

Signed-off-by: Akashi Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>