cert_create: specify command line options in the CoT

This patch introduces a new API that allows to specify command
line options in the Chain of Trust description. These command line
options may be used to specify parameters related to the CoT (i.e.
keys or certificates), instead of keeping a hardcoded list of
options in main.c.

Change-Id: I282b0b01cb9add557b26bddc238a28253ce05e44
diff --git a/tools/cert_create/include/key.h b/tools/cert_create/include/key.h
index da9f119..6995a06 100644
--- a/tools/cert_create/include/key.h
+++ b/tools/cert_create/include/key.h
@@ -63,12 +63,15 @@
  */
 typedef struct key_s {
 	int id;			/* Key id */
+	const char *opt;	/* Command line option to specify a key */
 	const char *desc;	/* Key description (debug purposes) */
 	char *fn;		/* Filename to load/store the key */
 	EVP_PKEY *key;		/* Key container */
 } key_t;
 
 /* Exported API */
+int key_init(void);
+key_t *key_get_by_opt(const char *opt);
 int key_create(key_t *key, int type);
 int key_load(key_t *key, unsigned int *err_code);
 int key_store(key_t *key);