[rng] Allow entropy_enable() to return an error

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

src/arch/i386/interface/pcbios/rtc_entropy.c

 /**
  * Enable entropy gathering
  *
+ * @ret rc		Return status code
  */
-static void rtc_entropy_enable ( void ) {
+static int rtc_entropy_enable ( void ) {
 
 	rtc_hook_isr();
 	enable_irq ( RTC_IRQ );
 	rtc_enable_int();
+
+	return 0;
 }
 
 /**

src/crypto/entropy.c

 	int rc;
 
 	/* Enable entropy gathering */
-	entropy_enable();
+	if ( ( rc = entropy_enable() ) != 0 )
+		return rc;
 
 	/* Perform mandatory startup tests, if not yet performed */
 	for ( ; startup_tested < startup_test_count() ; startup_tested++ ) {

src/include/ipxe/entropy.h

 /**
  * 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

 #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

src/tests/entropy_sample.c

 	for ( i = 0 ; i < ( SAMPLE_COUNT / SAMPLE_BLOCKSIZE ) ; i++ ) {
 
 		/* Collect one block of samples */
-		entropy_enable();
+		rc = entropy_enable();
+		ok ( rc == 0 );
 		for ( j = 0 ; j < SAMPLE_BLOCKSIZE ; j++ ) {
 			rc = get_noise ( &samples[j] );
 			ok ( rc == 0 );