[settings] Add "unixtime" builtin setting to expose the current time

Expose the current wall-clock time (in seconds since the Epoch), since
this is often useful in captured boot logs and can also be useful when
checking unexpected X.509 certificate validation failures.

Use a :uint32 setting to avoid Y2K38 rollover, thereby ensuring that
this will eventually be somebody else's problem.

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

src/core/settings.c

 #include <byteswap.h>
 #include <errno.h>
 #include <assert.h>
+#include <time.h>
 #include <ipxe/in.h>
 #include <ipxe/ip.h>
 #include <ipxe/ipv6.h>
 	.fetch = version_fetch,
 };
 
+/**
+ * Fetch current time setting
+ *
+ * @v data		Buffer to fill with setting data
+ * @v len		Length of buffer
+ * @ret len		Length of setting data, or negative error
+ */
+static int unixtime_fetch ( void *data, size_t len ) {
+	uint32_t content;
+
+	/* Return current time */
+	content = htonl ( time(NULL) );
+	if ( len > sizeof ( content ) )
+		len = sizeof ( content );
+	memcpy ( data, &content, len );
+	return sizeof ( content );
+}
+
+/** Current time setting */
+const struct setting unixtime_setting __setting ( SETTING_MISC, unixtime ) = {
+	.name = "unixtime",
+	.description = "Seconds since the Epoch",
+	.type = &setting_type_uint32,
+	.scope = &builtin_scope,
+};
+
+/** Current time built-in setting */
+struct builtin_setting unixtime_builtin_setting __builtin_setting = {
+	.setting = &unixtime_setting,
+	.fetch = unixtime_fetch,
+};
+
 /**
  * Fetch built-in setting
  *