Add config macro for min bytes hw entropy

ChangeLog

@@ -12,6 +12,9 @@ Features
      calling mbedtls_x509_crt_parse_path().
    * Added a configuration file config-no-entropy.h that enables a subset of
      library features that do not require an entropy source.
+   * Added the macro MBEDTLS_ENTROPY_MIN_HARDWARE in config.h. This allows users
+     to configure the minimum number of bytes for entropy sources using the
+     mbedtls_hardware_poll() function.
 
 Bugfix
    * Fix for platform time abstraction to avoid dependency issues where a build

configs/config-no-entropy.h

@@ -22,7 +22,8 @@
  * Minimal configuration of features that do not require an entropy source
  * Distinguishing reatures:
  * - no entropy module
- * - no TLS protocol implementation available due missing entropy source
+ * - no TLS protocol implementation available due to absence of an entropy
+ *   source
  *
  * See README.txt for usage instructions.
  */

include/mbedtls/config.h

@@ -2509,6 +2509,7 @@
 /* Entropy options */
 //#define MBEDTLS_ENTROPY_MAX_SOURCES                20 /**< Maximum number of sources supported */
 //#define MBEDTLS_ENTROPY_MAX_GATHER                128 /**< Maximum amount requested from entropy sources */
+//#define MBEDTLS_ENTROPY_MIN_HARDWARE               32 /**< Default minimum number of bytes required for the hardware entropy source mbedtls_hardware_poll() before entropy is released */
 
 /* Memory buffer allocator options */
 //#define MBEDTLS_MEMORY_ALIGN_MULTIPLE      4 /**< Align on multiples of this value */

include/mbedtls/entropy_poll.h

@@ -41,7 +41,9 @@ extern "C" {
 #define MBEDTLS_ENTROPY_MIN_PLATFORM     32     /**< Minimum for platform source    */
 #define MBEDTLS_ENTROPY_MIN_HAVEGE       32     /**< Minimum for HAVEGE             */
 #define MBEDTLS_ENTROPY_MIN_HARDCLOCK     4     /**< Minimum for mbedtls_timing_hardclock()        */
+#if !defined(MBEDTLS_ENTROPY_MIN_HARDWARE)
 #define MBEDTLS_ENTROPY_MIN_HARDWARE     32     /**< Minimum for the hardware source */
+#endif
 
 /**
  * \brief           Entropy poll callback that provides 0 entropy.