classes/rootfs_*: error out if postinstalls exist with read-only rootfs

classes/rootfs_*: error out if postinstalls exist with read-only rootfs

If there are postinstall scripts to be run on first boot and
IMAGE_FEATURES contains "read-only-rootfs", we know this will fail on
the target device so just error out during do_rootfs. This check was
already implemented for ipk, so add it to the deb and rpm backends. Also
make all backends use bberror rather than just echo to display the
error.

Fixes [YOCTO #3407].

(From OE-Core rev: 510e377f252dece8850487c30482f83df4a89c7e)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>