libc: Import strtoul 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: I8c3b92751d1ce226c966f7c81fedd83f0846865e
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/include/lib/libc/stdlib.h b/include/lib/libc/stdlib.h
index b4a14e8..3b5e915 100644
--- a/include/lib/libc/stdlib.h
+++ b/include/lib/libc/stdlib.h
@@ -26,4 +26,5 @@
 extern void exit(int status);
 
 long strtol(const char *nptr, char **endptr, int base);
+unsigned long strtoul(const char *nptr, char **endptr, int base);
 #endif /* STDLIB_H */