fix(cert-create): fix key loading logic

When key_load() attempts to load the key from a file and it fails to
open this file, the 'err_code' output argument is set to
'KEY_ERR_OPEN' error code. However, it is incorrectly overwritten
later on with 'KEY_ERR_NONE' or 'KEY_ERR_LOAD'.

The latter case messes up with the key creation scenario. The
'KEY_ERR_LOAD' error leads the tool to exit, when it should attempt to
create the said key file if invoked with the --new-keys/-n option.

Note that, to complicate matters further, which of 'KEY_ERR_OPEN' or
'KEY_ERR_NONE' values is returned by key_load() depends on the version
of OpenSSL in use:

 - If using v3+, KEY_ERROR_LOAD is returned.

 - If using <v3, KEY_ERROR_NONE is returned as a result of the key
   pair container being initialized by key_new().

This patch fixes this bug and also takes the opportunity to refactor
key_load() implementation to (hopefully) make it more straight-forward
and easier to reason about.

Fixes: 616b3ce27d9a "feat(cert-create): add pkcs11 engine support"
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Reported-by: Wenchen Tan <xtaens@qq.com>
Change-Id: Ia78ff442e04c5ff98e6ced8d26becbd817a8ccb7
diff --git a/tools/cert_create/include/key.h b/tools/cert_create/include/key.h
index 56f1c21..e0ecdae 100644
--- a/tools/cert_create/include/key.h
+++ b/tools/cert_create/include/key.h
@@ -74,7 +74,7 @@
 int key_new(key_t *key);
 #endif
 int key_create(key_t *key, int type, int key_bits);
-int key_load(key_t *key, unsigned int *err_code);
+unsigned int key_load(key_t *key);
 int key_store(key_t *key);
 void key_cleanup(void);