lib: add stdlib.c

Since we need to realize standard library function other than use them
with gcc tool chain in U-Boot. So add standard library function here.

Change-Id: I10009c5bbe31fabacd929df3c44218ae9c6a885f
Signed-off-by: Jason Zhu <jason.zhu@rock-chips.com>

include/stdlib.h

@@ -9,4 +9,6 @@
 
 #include <malloc.h>
 
+int atoi(const char *nptr);
+
 #endif /* __STDLIB_H_ */

lib/Makefile

@@ -90,6 +90,7 @@ obj-y += linux_string.o
 obj-y += membuff.o
 obj-$(CONFIG_REGEX) += slre.o
 obj-y += string.o
+obj-y += stdlib.o
 obj-y += tables_csum.o
 obj-y += time.o
 obj-$(CONFIG_TRACE) += trace.o

lib/stdlib.c

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2019 Fuzhou Rockchip Electronics Co., Ltd
+ */
+
+#include <linux/ctype.h>
+#include <linux/types.h>
+
+long atol(const char *nptr)
+{
+	int c;
+	long total;
+	int sign;
+
+	while (isspace((int)(unsigned char)*nptr))
+		++nptr;
+
+	c = (int)(unsigned char)*nptr++;
+	sign = c;
+	if (c == '-' || c == '+')
+		c = (int)(unsigned char)*nptr++;
+
+	total = 0;
+
+	while (isdigit(c)) {
+		total = 10 * total + (c - '0');
+		c = (int)(unsigned char)*nptr++;
+	}
+
+	if (sign == '-')
+		return -total;
+	else
+		return total;
+}
+
+int atoi(const char *nptr)
+{
+	return (int)atol(nptr);
+}