KEYS: trusted: improve scalability of trust source config

Enabling trusted keys requires at least one trust source implementation
(currently TPM, TEE or CAAM) to be enabled. Currently, this is
done by checking each trust source's config option individually.
This does not scale when more trust sources like the one for DCP
are added, because the condition will get long and hard to read.

Add config HAVE_TRUSTED_KEYS which is set to true by each trust source
once its enabled and adapt the check for having at least one active trust
source to use this option. Whenever a new trust source is added, it now
needs to select HAVE_TRUSTED_KEYS.

Signed-off-by: David Gstir <[e-mail]>
Tested-by: Jarkko Sakkinen <[e-mail]> # for TRUSTED_KEYS_TPM
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>

security/keys/trusted-keys/Kconfig

···
+config HAVE_TRUSTED_KEYS
+	bool
+
 config TRUSTED_KEYS_TPM
 	bool "TPM-based trusted keys"
 	depends on TCG_TPM >= TRUSTED_KEYS
···
 	select ASN1_ENCODER
 	select OID_REGISTRY
 	select ASN1
+	select HAVE_TRUSTED_KEYS
 	help
 	  Enable use of the Trusted Platform Module (TPM) as trusted key
 	  backend. Trusted keys are random number symmetric keys,
···
 	bool "TEE-based trusted keys"
 	depends on TEE >= TRUSTED_KEYS
 	default y
+	select HAVE_TRUSTED_KEYS
 	help
 	  Enable use of the Trusted Execution Environment (TEE) as trusted
 	  key backend.
···
 	depends on CRYPTO_DEV_FSL_CAAM_JR >= TRUSTED_KEYS
 	select CRYPTO_DEV_FSL_CAAM_BLOB_GEN
 	default y
+	select HAVE_TRUSTED_KEYS
 	help
 	  Enable use of NXP's Cryptographic Accelerator and Assurance Module
 	  (CAAM) as trusted key backend.
 
-if !TRUSTED_KEYS_TPM && !TRUSTED_KEYS_TEE && !TRUSTED_KEYS_CAAM
-comment "No trust source selected!"
+if !HAVE_TRUSTED_KEYS
+	comment "No trust source selected!"
 endif