boards/esp32c3-devkit: add Dropbear SSH server configuration

[FelipeMdeO]

Summary

This PR adds two related changes that together bring up an SSH server
on the ESP32-C3 DevKit board using the Dropbear application:

boards/risc-v/esp32c3/esp32c3-devkit/configs/dropbear

A new dropbear defconfig is introduced for the ESP32-C3 DevKit board.
It wires up the Dropbear SSH server application together with:

• Wi-Fi STA mode with DHCP client (WAPI tooling included for link
  bring-up at boot).
• urandom device for key material.
• SPIFFS on SPI flash (/data mountpoint) to persist the host key and
  the password database.
• FSUTILS_PASSWD pointing to /data/passwd as the credential store,
  replacing a previous Dropbear-specific password-file path.
• ECDSA host key stored at /data/dropbear_ecdsa_host_key.
• NSH autostart of the dropbear task on every boot.
• PTY support and Ctrl-C signal delivery enabled for interactive
  sessions.
• CONFIG_NETUTILS_DROPBEAR_STACKSIZE pinned to 65536 bytes; the
  default 32 KiB overflows during key exchange on this RISC-V target.
• CONFIG_NETUTILS_DROPBEAR_LISTEN_RETRY_MAX=120 so the daemon keeps
  retrying until the Wi-Fi link is fully up.

Wi-Fi credentials (myssid / mypasswd) - Please remember give some connectivity to board.

crypto: expose ChaCha20 stream helpers

Dropbear uses the chacha20-poly1305@openssh.com cipher, which requires
a stateful, multi-call ChaCha20 stream interface rather than the single-
block interface currently exposed by crypto/chachapoly.c. Three helpers
and a context struct are added:

• struct chacha20_stream_ctx — opaque wrapper around chacha_ctx.
• chacha20_stream_setkey() — initialise the key.
• chacha20_stream_ivctr64() — set IV and 64-bit counter.
• chacha20_stream_crypt() — encrypt/decrypt an arbitrary-length buffer.

All three functions are thin wrappers over the existing chacha_*
primitives; no new algorithm code is introduced.

Impact

• New board configuration: the dropbear defconfig is additive and
  does not affect any existing configuration.
• New public API: three functions and one struct are added to
  include/crypto/chachapoly.h. The change is purely additive; existing
  users of chacha20_setkey / chacha20_crypt are unaffected.
• Build: no impact on boards or configurations that do not select
  CONFIG_NETUTILS_DROPBEAR.
• Security: host keys and credentials live on a SPIFFS partition
  under /data; they are generated at first run and persist across
  reboots. Wi-Fi credentials must be provisioned by the user before
  flashing.

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)
make flash ESPTOOL_PORT=/dev/ttyUSB0

Boot log shows Dropbear listening after Wi-Fi association:

NuttShell (NSH) NuttX-12.x
nsh> Starting Dropbear SSH server
Dropbear listening on port 22

SSH connection from the host:

$ ssh -o StrictHostKeyChecking=no user@<board-ip>
user@<board-ip>'s password: <passwd>
NuttShell (NSH) NuttX-12.x
nsh>

[fdcavalcanti]

Very cool!
Please add documentation for this defconfig: Documentation/platforms/risc-v/esp32c3.