wic: add 'part-name' argument for naming GPT partitions
The WIC's 'part' can now give a name for GPT partition in WKS file.
It's similar to '--label', but is naming partintions instead file systems.
It's required by some bootloaders to partitions have specified names.
Signed-off-by: Artur Mądrzak <artur@madrzak.eu> Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
