libc: Import strtol from FreeBSD project

From commit: 21571b1d140ae7bb44e94c0afba2ec61456b275b
The coding guidelines[1] in TF-A forbid the use of ato*() functions
in favour of strto*(). However, the TF-A libc does not provide an
implementation of strto*(), making this rule impossible to satisfy.

Also made small changes to fit into TF-A project. Added the source
files to the libc makefile

[1] https://trustedfirmware-a.readthedocs.io/en/latest/process/coding-guidelines.html#libc-functions-that-are-banned-or-to-be-used-with-caution

Change-Id: Ica95bf5da722913834fe90bf3fe743aa34e01e80
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/include/lib/libc/stdlib.h b/include/lib/libc/stdlib.h
index 24e7bae..b4a14e8 100644
--- a/include/lib/libc/stdlib.h
+++ b/include/lib/libc/stdlib.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012-2017 Roberto E. Vargas Caballero
+ * Copyright (c) 2012-2021 Roberto E. Vargas Caballero
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -18,8 +18,12 @@
 
 #define _ATEXIT_MAX 1
 
+#define isspace(x)    (((x) == ' ') || ((x) == '\r') || ((x) == '\n') || \
+			((x) == '\t') || ((x) == '\b'))
+
 extern void abort(void);
 extern int atexit(void (*func)(void));
 extern void exit(int status);
 
+long strtol(const char *nptr, char **endptr, int base);
 #endif /* STDLIB_H */