Introduce platform-layer functions for reading/writing seed from NV

Introduces mbedtls_nv_seed_read() and mbedtls_nv_seed_write().

The platform-layer functions are only available when
MBEDTLS_ENTROPY_NV_SEED is enabled.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index f800f13..3e12f43 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -156,6 +156,7 @@
 //#define MBEDTLS_PLATFORM_FPRINTF_ALT
 //#define MBEDTLS_PLATFORM_PRINTF_ALT
 //#define MBEDTLS_PLATFORM_SNPRINTF_ALT
+//#define MBEDTLS_PLATFORM_NV_SEED_ALT
 
 /**
  * \def MBEDTLS_DEPRECATED_WARNING
@@ -825,6 +826,25 @@
 //#define MBEDTLS_ENTROPY_FORCE_SHA256
 
 /**
+ * \def MBEDTLS_ENTROPY_NV_SEED
+ *
+ * Enable the non-volatile (NV) seed file-based entropy source.
+ * (Also enables the NV seed read/write functions in the platform layer)
+ *
+ * This is crucial (if not required) on systems that do not have a
+ * cryptographic entropy source (in hardware or kernel) available.
+ *
+ * Requires: MBEDTLS_ENTROPY_C, MBEDTLS_PLATFORM_C
+ *
+ * Note: If you use the default implementation functions that read a seedfile
+ *       with regular fopen(), please make sure you make a seedfile with the
+ *       proper name (defined in MBEDTLS_PLATFORM_STD_NV_SEED_FILE) and at
+ *       least MBEDTLS_ENTROPY_BLOCK_SIZE bytes in size that can be read from
+ *       and written to or you will get an entropy source error!
+ */
+//#define MBEDTLS_ENTROPY_NV_SEED
+
+/**
  * \def MBEDTLS_MEMORY_DEBUG
  *
  * Enable debugging of buffer allocator memory issues. Automatically prints
@@ -2498,6 +2518,9 @@
 //#define MBEDTLS_PLATFORM_STD_SNPRINTF    snprintf /**< Default snprintf to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_EXIT_SUCCESS       0 /**< Default exit value to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_EXIT_FAILURE       1 /**< Default exit value to use, can be undefined */
+//#define MBEDTLS_PLATFORM_STD_NV_SEED_READ   mbedtls_platform_std_nv_seed_read /**< Default nv_seed_read function to use, can be undefined */
+//#define MBEDTLS_PLATFORM_STD_NV_SEED_WRITE  mbedtls_platform_std_nv_seed_write /**< Default nv_seed_write function to use, can be undefined */
+//#define MBEDTLS_PLATFORM_STD_NV_SEED_FILE  "seedfile" /**< Seed file to read/write with default implementation */
 
 /* To Use Function Macros MBEDTLS_PLATFORM_C must be enabled */
 /* MBEDTLS_PLATFORM_XXX_MACRO and MBEDTLS_PLATFORM_XXX_ALT cannot both be defined */
@@ -2510,6 +2533,8 @@
 //#define MBEDTLS_PLATFORM_PRINTF_MACRO        printf /**< Default printf macro to use, can be undefined */
 /* Note: your snprintf must correclty zero-terminate the buffer! */
 //#define MBEDTLS_PLATFORM_SNPRINTF_MACRO    snprintf /**< Default snprintf macro to use, can be undefined */
+//#define MBEDTLS_PLATFORM_NV_SEED_READ_MACRO   mbedtls_platform_std_nv_seed_read /**< Default nv_seed_read function to use, can be undefined */
+//#define MBEDTLS_PLATFORM_NV_SEED_WRITE_MACRO  mbedtls_platform_std_nv_seed_write /**< Default nv_seed_write function to use, can be undefined */
 
 /* SSL Cache options */
 //#define MBEDTLS_SSL_CACHE_DEFAULT_TIMEOUT       86400 /**< 1 day  */