netutils/dropbear: initial Dropbear SSH server port for NuttX

[FelipeMdeO]

Summary

This PR is the first step of the Dropbear port to Apache NuttX.

At this stage, the port still uses Dropbear’s internal cryptographic library. Only the password-checking routines have been adapted to use native NuttX mechanisms.

As a second step, this application will be improved to use the cryptographic libraries provided by NuttX instead of Dropbear’s own crypto implementation.

With this application, a user can establish SSH connections to the hardware, allowing remote access to the system.

The documentation is already added to the mainline: PR#19190
Also, the defconfig to you be able test is available in the following commit: apache/nuttx#19062

This PR is a "reboot" of #3532. The first try added step 1 and 2 in the same PR adding a long discussion, so I drop the old PR and opened a clean PR here, ready to review.

Impact

New board configuration: the dropbear defconfig is additive and does not affect any existing configuration.
Is not expected any issue added by this code.

Testing

Host: Linux x86_64, GCC RISC-V toolchain
Board: ESP32-C3 DevKit (rev 0.4)

Build:

./tools/configure.sh esp32c3-devkit:dropbear
make -j$(nproc)
<setup wifi ssid and pass>
make flash ESPTOOL_PORT=/dev/ttyUSB0

First-time user provisioning (serial console):

The NuttX passwd file lives on SPIFFS (/data/passwd) and is empty on a
fresh flash. Before the first SSH login, create a user from the NSH
serial console:

nsh> useradd root <password>

The ECDSA host key is generated automatically on first boot.

Boot log shows Dropbear listening after Wi-Fi association:

NuttShell (NSH) NuttX-12.6.0
nsh> loaded ECDSA P-256 host key from /data/dropbear_ecdsa_host_key
     using NuttX passwd auth at /data/passwd
     dropbear: listening on port 2222

SSH connection from the host:

$ ssh -p 2222 root@<board-ip>
root@<board-ip>'s password: <password>
NuttShell (NSH) NuttX-12.6.0
nsh>

[FelipeMdeO]

@xiaoxiang781216, @acassis , @linguini1 , I dropped PR #3532 and this take its place.