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/lib/libc/libc_asm.mk b/lib/libc/libc_asm.mk
index 1597da4..e959687 100644
--- a/lib/libc/libc_asm.mk
+++ b/lib/libc/libc_asm.mk
@@ -26,6 +26,7 @@
 			strnlen.c			\
 			strrchr.c			\
 			strtok.c			\
+			strtoul.c			\
 			strtol.c)
 
 ifeq (${ARCH},aarch64)