image.bbclass: add a hook funtion to support readonly rootfs
read_only_rootfs_hook: this funtion is invoked only if 'read-only-rootfs'
is in IMAGE_FEATURES. It tweaks some basic files and makes populate-
volatile.sh script run at rootfs time.
For now, it has real effect only if the init system is sysvinit.
[YOCTO #3406]
(From OE-Core rev:
cae51a169e37b54becc08162aaa643acd53ffe2f)
Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>