base-files: Make sure rootfs_data_max is considered

base-files: Make sure rootfs_data_max is considered

For sysupgrade on NAND/UBI devices there is the U-Boot environment
variable rootfs_data_max which can be used to limit the size of the
rootfs_data volume created on sysupgrade.
This stopped working reliable with recent kernels, probably due to a
race condition when reading the number of free erase blocks from sysfs
just after removing a volume.
Change the script to just try creating rootfs_data with the desired
size and retry with maximum size in case that fails. Hence calculating
the available size in the script can be dropped which works around the
problem.

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