Move SHA256_VALIDATE[_RET] outside of MBEDTLS_SHA256_ALT guard

Somehow, mbedtls_sha256_ret() is defined even if MBEDTLS_SHA256_ALT
is set, and it is using SHA256_VALIDATE_RET. The documentation should
be enhanced to indicate that MBEDTLS_SHA256_ALT does _not_ replace
the entire module, but only the core SHA-256 functions.

library/sha256.c

diff --git a/library/sha256.c b/library/sha256.c
index 9967d52..8a540ad 100644
--- a/library/sha256.c
+++ b/library/sha256.c
@@ -49,6 +49,10 @@
 #endif /* MBEDTLS_PLATFORM_C */
 #endif /* MBEDTLS_SELF_TEST */
 
+#define SHA256_VALIDATE_RET(cond)                           \
+    MBEDTLS_INTERNAL_VALIDATE_RET( cond, MBEDTLS_ERR_SHA256_BAD_INPUT_DATA )
+#define SHA256_VALIDATE(cond)  MBEDTLS_INTERNAL_VALIDATE( cond )
+
 #if !defined(MBEDTLS_SHA256_ALT)
 
 /*
@@ -74,10 +78,6 @@
 } while( 0 )
 #endif
 
-#define SHA256_VALIDATE_RET(cond)                           \
-    MBEDTLS_INTERNAL_VALIDATE_RET( cond, MBEDTLS_ERR_SHA256_BAD_INPUT_DATA )
-#define SHA256_VALIDATE(cond)  MBEDTLS_INTERNAL_VALIDATE( cond )
-
 void mbedtls_sha256_init( mbedtls_sha256_context *ctx )
 {
     SHA256_VALIDATE( ctx != NULL );