ipq806x: onhub: Enable fstools_partname_fallback_scan

ipq806x: onhub: Enable fstools_partname_fallback_scan

When fstools is unable to parse our root=<...> arg correctly, it can
fall back to scanning all block devices for a 'rootfs_data' partition.
This fallback was deemed wrong (or at least, a breaking/incompatible
change) for some targets, so we're forced to opt back into it with
fstools_partname_fallback_scan=1.

Without this, OnHub devices will use a rootfs-appended loop device for
rootfs_data instead of the intended 3rd partition.

While I'm at it, just move all the boot args into the 'cros-vboot'
build rule, instead of using the custom bootargs-append. All cros-vboot
subtargets here are using the same rootwait (to support both eMMC and
USB boot) and root/partition args.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
[ drop unrelated comments in commit description ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>