[lib][libc] add simple errno implementation

Completely un-threadsafe, but enough to satisfy the __geterrno extern.

Change-Id: I95b23f381767a3d55f43d77c188cb0cd611536ee

lib/libc/errno.c

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2015 Travis Geiselbrecht
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files
+ * (the "Software"), to deal in the Software without restriction,
+ * including without limitation the rights to use, copy, modify, merge,
+ * publish, distribute, sublicense, and/or sell copies of the Software,
+ * and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+ * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+ * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+ * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+ * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include <errno.h>
+
+/* completely un-threadsafe implementation of errno */
+/* TODO: pull from kernel TLS or some other thread local storage */
+static int _errno;
+
+int *__geterrno(void)
+{
+    return &_errno;
+}
+

lib/libc/rules.mk

@@ -6,6 +6,7 @@ MODULE_SRCS += \
 	$(LOCAL_DIR)/atoi.c \
 	$(LOCAL_DIR)/bsearch.c \
 	$(LOCAL_DIR)/ctype.c \
+	$(LOCAL_DIR)/errno.c \
 	$(LOCAL_DIR)/printf.c \
 	$(LOCAL_DIR)/rand.c \
 	$(LOCAL_DIR)/stdio.c \