libc: minimal: Add error codes for key management

This adds EKEY* codes, as defined by Linux, using the same numeric
values as Linux.

Signed-off-by: Nils Montenegro <nils.montenegro@nordicsemi.no>

lib/libc/minimal/include/errno.h

@@ -126,6 +126,12 @@ extern "C" {
 #define ETIME 79   /* STREAMS timeout occurred */
 #define ENOMSG 80  /* Unexpected message type */
 
+/* specific encryption errno values */
+#define ENOKEY 126       /* Required key not available */
+#define EKEYEXPIRED 127  /* Required key not available */
+#define EKEYREVOKED 128  /* Key has been revoked */
+#define EKEYREJECTED 129 /* Key was rejected by service */
+
 #define EILSEQ 138 /* Illegal byte sequence */
 
 #ifdef __cplusplus