[console] Add "log message" console usage and an internal syslog() call

Provide an internal syslog() function (unrelated to the syslog
console) which can be used to create log messages with specified
priorities.

The build-time constant LOG_LEVEL can be used to select the minimum
required priority for log messages.  Any messages that do not have a
sufficient priority will be ignored (and will be optimised away at
compile-time).

The default LOG_LEVEL is LOG_NONE.

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

src/core/log.c

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2012 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+/** @file
+ *
+ * System logger
+ *
+ */
+
+#include <stdarg.h>
+#include <syslog.h>
+#include <ipxe/console.h>
+
+/**
+ * Write message to system log
+ *
+ * @v fmt		Format string
+ * @v args		Arguments
+ */
+void log_vprintf ( const char *fmt, va_list args ) {
+	int saved_usage;
+
+	/* Mark console as in use for log messages */
+	saved_usage = console_set_usage ( CONSOLE_USAGE_LOG );
+
+	/* Print message */
+	vprintf ( fmt, args );
+
+	/* Restore console usage */
+	console_set_usage ( saved_usage );
+}
+
+/**
+ * Write message to system log
+ *
+ * @v fmt		Format string
+ * @v ...		Arguments
+ */
+void log_printf ( const char *fmt, ... ) {
+	va_list args;
+
+	va_start ( args, fmt );
+	log_vprintf ( fmt, args );
+	va_end ( args );
+}