[time] Allow system clock to be adjusted at runtime

Provide a mechanism to allow an arbitrary adjustment to be applied to
all subsequent calls to time().

Note that the underlying clock source (e.g. the RTC clock) will not be
changed; only the time as reported within iPXE will be affected.

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

src/core/time.c

  * 400.
  */
 
+/** Current system clock offset */
+signed long time_offset;
+
 /** Days of week (for debugging) */
 static const char *weekdays[] = {
 	"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"

src/include/ipxe/time.h

 /* Include all architecture-dependent time API headers */
 #include <bits/time.h>
 
+extern signed long time_offset;
+
 /**
- * Get current time in seconds
+ * Get current time in seconds (ignoring system clock offset)
  *
  * @ret time		Time, in seconds
  */
 time_t time_now ( void );
 
+/**
+ * Adjust system clock
+ *
+ * @v delta		Clock adjustment, in seconds
+ */
+static inline __attribute__ (( always_inline )) void
+time_adjust ( signed long delta ) {
+
+	time_offset += delta;
+}
+
 #endif /* _IPXE_TIME_H */

src/include/time.h

  * @v t			Time to fill in, or NULL
  * @ret time		Current time
  */
-static inline time_t time ( time_t *t ) {
+static inline __attribute__ (( always_inline )) time_t time ( time_t *t ) {
 	time_t now;
 
-	now = time_now();
+	now = ( time_now() + time_offset );
 	if ( t )
 		*t = now;
 	return now;