init: ONSUCCESS={kexec|reboot|halt|console} support

When edeploy completed the installation, depending on the use case, we
could have different needs.

Some needs to do some kexec stuff which was handled by the NOREBOOT=1
option. Some other needs to halt, reboot or take a console.

To make all this use cases better integrated, the ONSUCCESS= variable
can be defined to tell edeploy what to do when install succeed.

Values are pretty trivial to understand :
- kexec
- reboot
- halt
- console

reboot is the default value to keep the same behavior.

NOREBOOT=1 becomes deprecated but a compatibility is kept by making an
implicit ONSUCCESS=kexec. This compatibility will be removed later once
users will have migrated their configuration. At least a warning is
shown.

Enjoy,