partname: allow skipping existing 'rootfs_data' partition

partname: allow skipping existing 'rootfs_data' partition

Apparently some devices with eMMC (e.g. ZyXEL NBG6817) got an exsiting
GPT partition called 'rootfs_data'. This partition is then selected
as rootfs_data eventhough it may not be suitable for that (too small)
or serve another purpose (ie. used by vendor firmware).
To avoid this, check if the kernel cmdline contains a variable
fstools_ignore_partname=1 and if so, skip partname detection of volumes
alltogether.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>