Correct node erase sequence

This sequence is more accurate as it sends SIGTERM to all processes.

- Send SIGTERM to all processes
- Trap SIGTERM
- Sync and set RO flag on all partitions
- Erase bootable partitions with dd
- Send "b" to sysrq-trigger

Closes-Bug: 1268641
Closes-Bug: 1279720

Change-Id: I4eaa6ed9b2872f55efaff0a874ac280bdba02226