mvebu: puzzle-m90x: wipe rootfs_data on sysupgrade

mvebu: puzzle-m90x: wipe rootfs_data on sysupgrade

The sysupgrade formware of the Puzzle series is a slightly strange
dual-boot approach while remaining compatible with Marvell's SDK
firmware upgrade binary format -- which happens to be a full-disk
image with GPT partition table. Hence that /lib/upgrade/emmc-puzzle.sh
script is like an exotic disease which results from those decisions,
and as we also want to somehow stay compatible with the IEI-World
stock firmware we got to use it in that same way (we are not
compatible with the QNAP-branded identical hardware device anyway).

Currently, on sysupgrade the result is that one ends up with the old
content of rootfs_data (a GPT partition on those devices) as nothing
ever wipes or in any way re-creates the filesystem there. As a simple
work-around, let's kill the filesystem on rootfs_data so fstools
re-formats it on the next boot.

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