Unify the example programs' termination

This is done to account for platforms, for which we want custom behavior
upon the program termination, hence we call `mbedtls_exit()` instead of
returning from `main()`.
diff --git a/programs/hash/generic_sum.c b/programs/hash/generic_sum.c
index 709a149..97f7a46 100644
--- a/programs/hash/generic_sum.c
+++ b/programs/hash/generic_sum.c
@@ -48,7 +48,7 @@
 int main( void )
 {
     mbedtls_printf("MBEDTLS_MD_C and/or MBEDTLS_FS_IO not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -203,7 +203,7 @@
         fflush( stdout ); getchar();
 #endif
 
-        return( exit_code );
+        mbedtls_exit( exit_code );
     }
 
     /*
@@ -213,12 +213,12 @@
     if( md_info == NULL )
     {
         mbedtls_fprintf( stderr, "Message Digest '%s' not found\n", argv[1] );
-        return( exit_code );
+        mbedtls_exit( exit_code );
     }
     if( mbedtls_md_setup( &md_ctx, md_info, 0 ) )
     {
         mbedtls_fprintf( stderr, "Failed to initialize context.\n" );
-        return( exit_code );
+        mbedtls_exit( exit_code );
     }
 
     ret = 0;
@@ -237,6 +237,6 @@
 exit:
     mbedtls_md_free( &md_ctx );
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_MD_C && MBEDTLS_FS_IO */
diff --git a/programs/hash/hello.c b/programs/hash/hello.c
index 55a0c7e..66fd315 100644
--- a/programs/hash/hello.c
+++ b/programs/hash/hello.c
@@ -44,7 +44,7 @@
 int main( void )
 {
     mbedtls_printf("MBEDTLS_MD5_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -58,7 +58,7 @@
     mbedtls_printf( "\n  MD5('%s') = ", str );
 
     if( ( ret = mbedtls_md5_ret( (unsigned char *) str, 13, digest ) ) != 0 )
-        return( MBEDTLS_EXIT_FAILURE );
+        mbedtls_exit( MBEDTLS_EXIT_FAILURE );
 
     for( i = 0; i < 16; i++ )
         mbedtls_printf( "%02x", digest[i] );
@@ -70,6 +70,6 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( MBEDTLS_EXIT_SUCCESS );
+    mbedtls_exit( MBEDTLS_EXIT_SUCCESS );
 }
 #endif /* MBEDTLS_MD5_C */