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/src/key.c b/tools/cert_create/src/key.c
index 76d528b..a7ee759 100644
--- a/tools/cert_create/src/key.c
+++ b/tools/cert_create/src/key.c
@@ -194,6 +194,7 @@
 
 int key_init(void)
 {
+	cmd_opt_t cmd_opt;
 	key_t *key;
 	int rc = 0;
 	unsigned int i;
@@ -201,11 +202,12 @@
 	for (i = 0; i < num_keys; i++) {
 		key = &keys[i];
 		if (key->opt != NULL) {
-			rc = cmd_opt_add(key->opt, required_argument,
-					 CMD_OPT_KEY);
-			if (rc != 0) {
-				break;
-			}
+			cmd_opt.long_opt.name = key->opt;
+			cmd_opt.long_opt.has_arg = required_argument;
+			cmd_opt.long_opt.flag = NULL;
+			cmd_opt.long_opt.val = CMD_OPT_KEY;
+			cmd_opt.help_msg = key->help_msg;
+			cmd_opt_add(&cmd_opt);
 		}
 	}