Added errno, strerror and the "%m" printf metacharacter. These will allow

us to return proper PXE status codes, while simultaneously allowing for
more consistent error reporting (complete with verbose error messages as a
build-time option).

src/core/errno.c

@@ -0,0 +1,29 @@
+#include "errno.h"
+#include "vsprintf.h"
+
+/* Global "last error" number */
+int errno;
+
+static struct errortab errortab_start[0] __table_start(errortab);
+static struct errortab errortab_end[0] __table_end(errortab);
+
+/*
+ * Retrieve string representation of error number.
+ *
+ * If error not found in the error table, generate a generic "Error
+ * 0x0000" message.
+ *
+ */
+const char * strerror ( int errno ) {
+	static char *generic_message = "Error 0x0000";
+	struct errortab *errortab;
+
+	for ( errortab = errortab_start ; errortab < errortab_end ;
+	      errortab++ ) {
+		if ( errortab->errno == errno )
+			return errortab->text;
+	}
+
+	sprintf ( generic_message + 8, "%hx", errno );
+	return generic_message;
+}