netutils/dropbear: retain child exit status for NSH PTY session

[FelipeMdeO]

Summary

Dropbear reaps the per-session NSH task with waitpid() in
dropbear_nshsession.c. Without CONFIG_SCHED_CHILD_STATUS the kernel does
not retain the child exit status, so waitpid() returns ECHILD right after
authentication and the interactive session never starts
(NSH session wait failed: Unknown error 10).
The issue was reported here: apache/nuttx#19209

Depend on SCHED_HAVE_PARENT (required by SCHED_CHILD_STATUS) and select
SCHED_CHILD_STATUS so any Dropbear build gets a working interactive session.

Impact

Kconfig-only, 2 lines. Fixes interactive SSH sessions for NETUTILS_DROPBEAR.
No dependency on other in-flight work.

Testing

Built sim:dropbear (nuttx master + this change) and connected over SSH:

on device side:

➜  ~ sudo setcap cap_net_admin+ep /home/felipe-moura/nuttx-space/pr-pure-test/nuttx/nuttx

[sudo] password for felipe-moura: 
➜  ~ cd /home/felipe-moura/nuttx-space/pr-pure-test/nuttx
./nuttx

dropbear [6:100]

NuttShell (NSH) NuttX-13.0.0
nsh> [6] Jun 01 00:00:00 using NuttX passwd auth at /tmp/passwd
dropbear: listening on port 2222
useradd felipe test1234
nsh> [6] Jun 01 00:00:24 connection from 10.0.1.1:54398
[6] Jun 01 00:00:33 Password auth succeeded for 'felipe' from 10.0.1.1:54398
[6] Jun 01 00:00:33 NSH PTY session started
ls
/:
 bin/
 dev/
 etc/
 proc/
 tmp/
 var/
nsh> poweroff
nsh: poweroff: command not found
nsh> 

On host side:

➜  ~ sudo ip addr add 10.0.1.1/24 dev tap0
sudo ip link set tap0 up

[sudo] password for felipe-moura: 
➜  ~ ssh-keygen -R '[10.0.1.2]:2222'
ssh -p 2222 felipe@10.0.1.2

# Host [10.0.1.2]:2222 found: line 9
/home/felipe-moura/.ssh/known_hosts updated.
Original contents retained as /home/felipe-moura/.ssh/known_hosts.old
The authenticity of host '[10.0.1.2]:2222 ([10.0.1.2]:2222)' can't be established.
ECDSA key fingerprint is SHA256:wRi61vyuX697dPhP5kny+tVZ1gyIgS7qgQ3s23PrQOk.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '[10.0.1.2]:2222' (ECDSA) to the list of known hosts.
felipe@10.0.1.2's password: 
nsh> ls
/:
 bin/
 dev/
 etc/
 proc/
 tmp/
 var/
nsh>

[FelipeMdeO]

Hello @acassis , @xiaoxiang781216 , @linguini1 , can you take a look, please?

[xiaoxiang781216]

let's change to depends on

[FelipeMdeO]

Hello @xiaoxiang781216, I'd prefer to keep this as select. With depends on, whoever writes a defconfig for a new board could forget to enable SCHED_CHILD_STATUS, and Dropbear would silently be dropped from the build, no error, it just wouldn't come up. Keeping it as select makes bringing up Dropbear on a new board simpler and avoids that pitfall.

[linguini1]

We did decide a while ago that we would use depends on going forward since select has a tendency to cause mysterious issues (not selecting recursive dependencies). The advised approach here is for the user to determine via Kconfig what dependencies need to be enabled and do that themselves.