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/check_config.h b/include/mbedtls/check_config.h
index 407cd57..63f93ec 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -3,7 +3,7 @@
  *
  * \brief Consistency checks for configuration options
  *
- *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
+ *  Copyright (C) 2006-2016, ARM Limited, All Rights Reserved
  *  SPDX-License-Identifier: Apache-2.0
  *
  *  Licensed under the Apache License, Version 2.0 (the "License"); you may
@@ -368,6 +368,38 @@
 #error "MBEDTLS_PLATFORM_STD_SNPRINTF defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_ENTROPY_NV_SEED) &&\
+    ( !defined(MBEDTLS_PLATFORM_C) || !defined(MBEDTLS_ENTROPY_C) )
+#error "MBEDTLS_ENTROPY_NV_SEED defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_NV_SEED_ALT) &&\
+    !defined(MBEDTLS_ENTROPY_NV_SEED)
+#error "MBEDTLS_PLATFORM_NV_SEED_ALT defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_STD_NV_SEED_READ) &&\
+    !defined(MBEDTLS_PLATFORM_NV_SEED_ALT)
+#error "MBEDTLS_PLATFORM_STD_NV_SEED_READ defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_STD_NV_SEED_WRITE) &&\
+    !defined(MBEDTLS_PLATFORM_NV_SEED_ALT)
+#error "MBEDTLS_PLATFORM_STD_NV_SEED_WRITE defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_NV_SEED_READ_MACRO) &&\
+    ( defined(MBEDTLS_PLATFORM_STD_NV_SEED_READ) ||\
+      defined(MBEDTLS_PLATFORM_NV_SEED_ALT) )
+#error "MBEDTLS_PLATFORM_NV_SEED_READ_MACRO and MBEDTLS_PLATFORM_STD_NV_SEED_READ cannot be defined simultaneously"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_NV_SEED_WRITE_MACRO) &&\
+    ( defined(MBEDTLS_PLATFORM_STD_NV_SEED_WRITE) ||\
+      defined(MBEDTLS_PLATFORM_NV_SEED_ALT) )
+#error "MBEDTLS_PLATFORM_NV_SEED_WRITE_MACRO and MBEDTLS_PLATFORM_STD_NV_SEED_WRITE cannot be defined simultaneously"
+#endif
+
 #if defined(MBEDTLS_RSA_C) && ( !defined(MBEDTLS_BIGNUM_C) ||         \
     !defined(MBEDTLS_OID_C) )
 #error "MBEDTLS_RSA_C defined, but not all prerequisites"