Remove MBEDTLS_ERR_SSL_CERTIFICATE_TOO_LARGE error code

This error is used when the output buffer isn't large enough
to hold our own certificate.

In the interest of cleaning up the error space for 3.0, this commit
removes MBEDTLS_ERR_SSL_CERTIFICATE_TOO_LARGE and replaces its single
use by MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 75faf22..422df99 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1964,7 +1964,7 @@
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "certificate too large, %" MBEDTLS_PRINTF_SIZET
                                         " > %" MBEDTLS_PRINTF_SIZET,
                            i + 3 + n, (size_t) MBEDTLS_SSL_OUT_CONTENT_LEN ) );
-            return( MBEDTLS_ERR_SSL_CERTIFICATE_TOO_LARGE );
+            return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );
         }
 
         ssl->out_msg[i    ] = (unsigned char)( n >> 16 );