Error codes for hardware accelerator failures

Add MBEDTLS_ERR_XXX_HW_ACCEL_FAILED error codes for all cryptography
modules where the software implementation can be replaced by a hardware
implementation.

This does not include the individual message digest modules since they
currently have no way to return error codes.

This does include the higher-level md, cipher and pk modules since
alternative implementations and even algorithms can be plugged in at
runtime.
diff --git a/include/mbedtls/xtea.h b/include/mbedtls/xtea.h
index 1d01e56..34ccee3 100644
--- a/include/mbedtls/xtea.h
+++ b/include/mbedtls/xtea.h
@@ -37,6 +37,7 @@
 #define MBEDTLS_XTEA_DECRYPT     0
 
 #define MBEDTLS_ERR_XTEA_INVALID_INPUT_LENGTH             -0x0028  /**< The data input has an invalid length. */
+#define MBEDTLS_ERR_XTEA_HW_ACCEL_FAILED                  -0x0029  /**< XTEA hardware accelerator failed. */
 
 #if !defined(MBEDTLS_XTEA_ALT)
 // Regular implementation