bcm27xx: Add padding after writing rootfs to image.

bcm27xx: Add padding after writing rootfs to image.

This addresses #9113 by adding up to 1MB padding after writing the
rootfs image. On boot mount_root will probe for existing filesystems
after the rootfs image data. Without overwriting the initial free
space left on the rootfs partition, OpenWrt might incorrectly detect
an exising filesystem and fails to mount it, resulting in a bricked
device as the overlayfs will not be mountend and settings will not be
available.

Fixes #9113.

Signed-off-by: Orne Brocaar <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19997
Signed-off-by: Robert Marko <[email protected]>