netutils/dropbear: back hmac-sha2-256 with NuttX /dev/crypto

[FelipeMdeO]

Summary

Replace Dropbear's bundled libtomcrypt HMAC modules with an adapter that
computes the hmac-sha2-256 packet MAC through the NuttX crypto device
(/dev/crypto) using CRYPTO_SHA2_256_HMAC sessions: hmac_init opens the
session with the MAC key, hmac_process feeds data with COP_FLAG_UPDATE
and hmac_done reads the tag and frees the session.

This PR is: espressif/esp-hal-3rdparty#13

The upstream Dropbear hmac_state is reused as-is, so no source patch is
needed: the existing hash field keeps the hash index and the /dev/crypto
session id is stored in the otherwise unused md buffer. The bundled
hmac_init.c/hmac_process.c/hmac_done.c are dropped from the build and
NETUTILS_DROPBEAR depends on CRYPTO_CRYPTODEV and
CRYPTO_CRYPTODEV_SOFTWARE_CRYPTO.

Only the HMAC is offloaded, not the plain SHA-256 hash: Dropbear's key
derivation (hashkeys() in common-kex.c) memcpy-clones a
partially-updated hash_state, which a kernel crypto session cannot
represent, so the SHA-256 descriptor stays on libtomcrypt.

Depends on #3636 (chacha20-poly1305 offload); this PR shares the Dropbear
Kconfig/Makefile/CMakeLists and will be rebased on top once that merges.
Opened as a draft to register the work.

Impact

• Users: none unless NETUTILS_DROPBEAR is enabled;

Testing

sim:dropbear on the NuttX simulator. Dropbear is started by NSH, a user is
created, and a host OpenSSH client connects forcing hmac-sha2-256 as the
MAC (with aes128-ctr, so the HMAC runs on every packet):

console Board side:

$ ./nuttx
dropbear: listening on port 2222
nsh> useradd admin nuttx2026
nsh> [6] Jun 01 00:00:15 connection from 172.17.0.2:53752
[6] Jun 01 00:00:15 Password auth succeeded for 'admin' from 172.17.0.2:53752
[6] Jun 01 00:00:15 NSH exec started: uname -a; ps

console PC side:

$ ssh -p 2222 -c aes128-ctr -m hmac-sha2-256 admin@10.0.1.2 'uname -a'
debug1: kex: server->client cipher: aes128-ctr MAC: hmac-sha2-256 compression: none
debug1: kex: client->server cipher: aes128-ctr MAC: hmac-sha2-256 compression: none
NuttX  12.6.0-RC1 sim sim