sysupgrade-nand: allow limiting rootfs_data by setting env variable

sysupgrade-nand: allow limiting rootfs_data by setting env variable

Check if firmware environment variable 'rootfs_data_max' exists and is
set to a numerical value greater than 0. If so, limit rootfs_data
volume to that size instead of using the maximum available size.

This is useful on devices with lots of flash where users may want to
have eg. a volume for persistent logs and statistics or for external
applications/containers. Persistence on rootfs overlay is limited by
the size of memory available during the sysugprade process as that
data needs to be copied to RAM while the volume is being recreated
during sysupgrade. Hence it is unsuitable for keeping larger amounts
of data accross upgrade which makes additional volume(s) for
application data desirable.

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