Support set *_drbg reseed interval before seed

mbedtls_ctr_drbg_set_reseed_interval() and
mbedtls_hmac_drbg_set_reseed_interval() can now be called before
their seed functions and the reseed_interval value will persist.
Previously it would be overwritten with the default value.

*_drbg_reseed_interval is now set in init() and free().

mbedtls_ctr_drbg_free() and mbedtls_hmac_drbg_free() now
reset the drbg context to the state immediately after init().

Tests:
- Added test to check that DRBG reseeds when reseed_counter
reaches reseed_interval, if reseed_interval set before seed
and reseed_interval is less than MBEDTLS_*_DRBG_RESEED_INTERVAL.

Signed-off-by: gacquroff <gavina352@gmail.com>

include/mbedtls/ctr_drbg.h

@@ -224,6 +224,11 @@ mbedtls_ctr_drbg_context;
  *                      and prepares it for mbedtls_ctr_drbg_seed()
  *                      or mbedtls_ctr_drbg_free().
  *
+ * \note                The reseed interval is
+ *                      #MBEDTLS_CTR_DRBG_RESEED_INTERVAL by default.
+ *                      You can override it by calling
+ *                      mbedtls_ctr_drbg_set_reseed_interval().
+ *
  * \param ctx           The CTR_DRBG context to initialize.
  */
 void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
@@ -305,7 +310,8 @@ int mbedtls_ctr_drbg_seed( mbedtls_ctr_drbg_context *ctx,
                    size_t len );
 
 /**
- * \brief               This function clears CTR_CRBG context data.
+ * \brief               This function resets CTR_DRBG context to the state immediately
+ *                      after initial call of mbedtls_ctr_drbg_init().
  *
  * \param ctx           The CTR_DRBG context to clear.
  */

include/mbedtls/hmac_drbg.h

@@ -138,6 +138,10 @@ typedef struct mbedtls_hmac_drbg_context
  * This function makes the context ready for mbedtls_hmac_drbg_seed(),
  * mbedtls_hmac_drbg_seed_buf() or mbedtls_hmac_drbg_free().
  *
+ * \note                The reseed interval is #MBEDTLS_HMAC_DRBG_RESEED_INTERVAL
+ *                      by default. Override this value by calling
+ *                      mbedtls_hmac_drbg_set_reseed_interval().
+ *
  * \param ctx           HMAC_DRBG context to be initialized.
  */
 void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
@@ -361,7 +365,8 @@ int mbedtls_hmac_drbg_random_with_add( void *p_rng,
 int mbedtls_hmac_drbg_random( void *p_rng, unsigned char *output, size_t out_len );
 
 /**
- * \brief               Free an HMAC_DRBG context
+ * \brief               This function resets HMAC_DRBG context to the state immediately
+ *                      after initial call of mbedtls_hmac_drbg_init().
  *
  * \param ctx           The HMAC_DRBG context to free.
  */