fix(tbbr): guard defines under MBEDTLS_CONFIG_FILE

Several platforms, such as NXP platforms, employ Trusted Boot support
without relying on MBEDTLS_CONFIG. This patch addresses the build
issues that arose on such platforms as a result of recent change
c1ec23dd60 [1].

[1]: https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/23730

Change-Id: Idfbeeafb8a30dc15bb0060beb5b17819a8807084
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>

include/common/tbbr/cot_def.h

@@ -32,7 +32,12 @@
  * buffers must be big enough to hold either. As RSA keys are bigger than ECDSA
  * ones for all key sizes we support, they impose the minimum size of these
  * buffers.
+ *
+ * If the platform employs its own mbedTLS configuration, it is the platform's
+ * responsibility to define TF_MBEDTLS_USE_RSA or TF_MBEDTLS_USE_ECDSA to
+ * establish the appropriate PK_DER_LEN size.
  */
+#ifdef MBEDTLS_CONFIG_FILE
 #if TF_MBEDTLS_USE_RSA
 #if TF_MBEDTLS_KEY_SIZE == 1024
 #define PK_DER_LEN                      162
@@ -66,5 +71,6 @@
 #else
 #error "Invalid value for TF_MBEDTLS_HASH_ALG_ID"
 #endif
+#endif /* MBEDTLS_CONFIG_FILE */
 
 #endif /* COT_DEF_H */