Remove noux from SSH exec terminal

This patch replaces the former Noux instance by a sub init that hosts
the combination of VFS + fs_rom + bash. If bash exits, the whole sub
init exits, which implicitly tears down the terminal session used by the
sub init. The latter is expected by the test sequence.

Issue #3696

tool/autopilot.list

@@ -48,6 +48,7 @@ rtc
 rump_ext2
 sd_card_bench
 seoul-auto
+ssh_exec_channel
 smartcard
 smbios_decoder
 smp