lib: charset: add u16_str<n>cmp()

u16 version of strcmp(): u16_strncmp() works like u16_strcmp() but only
at most n characters (in u16) are compared.
This function will be used in my UEFI secure boot patch.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>