cert_create: update help message

The help message printed by the cert_create tool using the command
line option -h (or --help) does not correctly list all the available
command line options.

This patch reworks the print_help() function to print the help
messages in a data driven approach. For each command line option
registered, an optional help message can be specified, which will
be printed by print_help().

Help messages for the TBBR options (certificates, keys and images)
are also provided.

Fix a small bug in the short options string passed to getopt_long:
the ':' was missing in the '-a' option (this option must take an
argument).

Fixes ARM-software/tf-issues#337

Change-Id: I9d08c2dfd349022808fcc884724f677eefdc1452
diff --git a/tools/cert_create/include/cert.h b/tools/cert_create/include/cert.h
index 11381c9..d38353a 100644
--- a/tools/cert_create/include/cert.h
+++ b/tools/cert_create/include/cert.h
@@ -57,6 +57,7 @@
 	const char *opt;	/* Command line option to pass filename */
 	const char *fn;		/* Filename to save the certificate */
 	const char *cn;		/* Subject CN (Company Name) */
+	const char *help_msg;	/* Help message */
 
 	/* These fields must be defined statically */
 	int key;		/* Key to be signed */