certs: Limit MODULE_SIG_KEY_TYPE_ECDSA to SHA384 or SHA512

NIST FIPS 186-5 states that it is recommended that the security
strength associated with the bit length of n and the security strength
of the hash function be the same, or higher upon agreement. Given NIST
P384 curve is used, force using either SHA384 or SHA512.
Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

certs/Kconfig

@@ -30,9 +30,11 @@ config MODULE_SIG_KEY_TYPE_RSA
 config MODULE_SIG_KEY_TYPE_ECDSA
 	bool "ECDSA"
 	select CRYPTO_ECDSA
+	depends on MODULE_SIG_SHA384 || MODULE_SIG_SHA512
 	help
-	 Use an elliptic curve key (NIST P384) for module signing. Consider
-	 using a strong hash like sha256 or sha384 for hashing modules.
+	 Use an elliptic curve key (NIST P384) for module signing. Use
+	 a strong hash of same or higher bit length, i.e. sha384 or
+	 sha512 for hashing modules.
 
 	 Note: Remove all ECDSA signing keys, e.g. certs/signing_key.pem,
 	 when falling back to building Linux 5.14 and older kernels.