libpayload: Add strerror

Change-Id: I33d45ad7d09473b8c6f5b7ee5fbadc0d184f9dcd
Signed-off-by: Stefan Tauner <stefan.tauner@gmx.at>
Reviewed-on: http://review.coreboot.org/3537
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.huber@secunet.com>

payloads/libpayload/include/string.h

@@ -65,6 +65,7 @@ size_t strspn(const char *s, const char *a);
 size_t strcspn(const char *s, const char *a);
 char *strtok(char *str, const char *delim);
 char *strtok_r(char *str, const char *delim, char **ptr);
+char *strerror(int errnum);
 /** @} */
 
 /**

payloads/libpayload/libc/string.c

@@ -639,3 +639,17 @@ void perror(const char *s)
 {
 	printf("%s: %d\n", s?s:"(none)", errno);
 }
+
+/**
+ * Get a message string describing the given error number.
+ *
+ * @param errnum The error number to be interpreted
+ * @return A pointer to a string describing the given error number
+ */
+char *strerror(int errnum)
+{
+	/* Reserve enough space for the string below + INT64_MIN in decimal + \0 */
+	static char errstr[35];
+	snprintf(errstr, sizeof(errstr), "Unknown error %d", errnum);
+	return errstr;
+}