kernel: add SOCK_RAW support and ICMP ping test

kernel: add SOCK_RAW support and ICMP ping test

- Add SOCK_RAW/IPPROTO_ICMP/IPPROTO_RAW constants to socket.h
- Extend ksocket struct with protocol field and raw_pcb pointer
- Add raw_recv_cb callback for receiving raw IP packets into ring buffer
- Extend ksocket_create/bind/send/sendto/recv/recvfrom/close/getsockname
  to handle SOCK_RAW using lwIP raw API
- Add O_NONBLOCK support to ksocket_recv (returns -EAGAIN instead of
  blocking forever) and merge fd flags in RECV/RECVFROM syscall handlers
- Add sys_sendto/sys_recvfrom wrappers and ICMP ping test (I7b) to fulltest
- Add ICMP ping test pattern to smoke_test.exp and test_battery.exp
- Migrate fstab parsing from kernel to userspace /sbin/init
- Auto-mount /disk and /persist in kernel init for init= binaries