TBBR: Reduce size of hash buffers when possible

The TBBR implementation extracts hashes from certificates and stores
them in static buffers. TF-A supports 3 variants of SHA right now:
SHA-256, SHA-384 and SHA-512. When support for SHA-512 was added in
commit 9a3088a5

 ("tbbr: Add build flag
HASH_ALG to let the user to select the SHA"), the hash buffers got
unconditionally increased from 51 to 83 bytes each. We can reduce that
space if we're using SHA-256 or SHA-384.

This saves some BSS space in both BL1 and BL2:
- BL1 with SHA-256: saving 168 bytes.
- BL1 with SHA-384: saving 80 bytes.
- BL2 with SHA-256: saving 384 bytes.
- BL2 with SHA-384: saving 192 bytes.

Change-Id: I0d02e5dc5f0162e82339c768609c9766cfe7e2bd
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

drivers/auth/tbbr/tbbr_cot.c

@@ -41,7 +41,15 @@
 #define PK_DER_LEN			91
 #endif
 
+#if TF_MBEDTLS_HASH_ALG_ID == TF_MBEDTLS_SHA256
+#define HASH_DER_LEN			51
+#elif TF_MBEDTLS_HASH_ALG_ID == TF_MBEDTLS_SHA384
+#define HASH_DER_LEN			67
+#elif TF_MBEDTLS_HASH_ALG_ID == TF_MBEDTLS_SHA512
 #define HASH_DER_LEN			83
+#else
+#error "Invalid value for TF_MBEDTLS_HASH_ALG_ID"
+#endif
 
 /*
  * The platform must allocate buffers to store the authentication parameters