[rng] Allow entropy_enable() to return an error

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/entropy.h

@@ -61,8 +61,9 @@ typedef uint8_t entropy_sample_t;
 /**
  * Enable entropy gathering
  *
+ * @ret rc		Return status code
  */
-void entropy_enable ( void );
+int entropy_enable ( void );
 
 /**
  * Disable entropy gathering

src/include/ipxe/null_entropy.h

@@ -19,9 +19,10 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #define ENTROPY_PREFIX_null __null_
 #endif
 
-static inline __always_inline void
+static inline __always_inline int
 ENTROPY_INLINE ( null, entropy_enable ) ( void ) {
 	/* Do nothing */
+	return 0;
 }
 
 static inline __always_inline void