Check if supplied rootfs_overlays have incompatible directories
Check if supplied rootfs_overlays have incompatible directories

Fixes https://github.com/nerves-project/nerves_system_br/issues/495

Some directories are mounted as filesystems on the device which overwrite
any files the overlay may have placed there. To prevent confusion, this will
detect these clashes and fail the firmware build with a helpful warning.

nerves-project/nerves,nerves-project/nerves