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]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
core-image.bbclass: support read-only rootfs

The hook function is appended to ROOTFS_POSTPROCESS_COMMAND
properly to support the 'read-only-rootfs' image feature.

[YOCTO #3406]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
populate-volatile.sh: improve this script

Here's a list of improvements:
1) Make it run correctly at rootfs time.
2) Handle link config items more reasonably.
3) Support read-only rootfs.
4) Avoid data loss when removing directories.

[YOCTO #3406]
[YOCTO #3404]
[YOCTO #3181]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>