module: Do not offer sha224 for built-in module signing

sha224 does not provide enough security against collision attacks
relative to the default keys used for signing (RSA 4k & P-384). Also
sha224 never became popular, as sha256 got widely adopter ahead of
sha224 being introduced.
Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

kernel/module/Kconfig

@@ -236,10 +236,6 @@ choice
 	  possible to load a signed module containing the algorithm to check
 	  the signature on that module.
 
-config MODULE_SIG_SHA224
-	bool "Sign modules with SHA-224"
-	select CRYPTO_SHA256
-
 config MODULE_SIG_SHA256
 	bool "Sign modules with SHA-256"
 	select CRYPTO_SHA256
@@ -257,7 +253,6 @@ endchoice
 config MODULE_SIG_HASH
 	string
 	depends on MODULE_SIG || IMA_APPRAISE_MODSIG
-	default "sha224" if MODULE_SIG_SHA224
 	default "sha256" if MODULE_SIG_SHA256
 	default "sha384" if MODULE_SIG_SHA384
 	default "sha512" if MODULE_SIG_SHA512