Provide bug4bug workaround for certain dumbiness of the u-boot's API_env_enum

function, which is expected to set returned env to NULL upon reaching the end
of the environment list but fails to do so in certain cases. The respective
u-boot code looks like the following (HEAD at the time of this commit):

--- api.c ---
 496 static int API_env_enum(va_list ap)
 ...
 510                 *next = last;
 511
 512                 for (i = 0; env_get_char(i) != '\0'; i = n + 1) {
 513                         for (n = i; env_get_char(n) != '\0'; ++n) {
 514                                 if (n >= CONFIG_ENV_SIZE) {
 515                                         /* XXX shouldn't we set *next = NULL?? */
 516                                         return 0;
 517                                 }
 518                         }
-------------

The net result is that any unfortunate user of the loader's ub_env_enum()
function hitting this condition would be trapped in the infinite loop, as
the main use pattern of ub_env_enum() is basically the following:

while ((env = ub_env_enum(env)) != NULL) { DO STUFF }

Which would stuck forever with the last element.

sys/boot/uboot/lib/glue.c

@@ -513,7 +513,7 @@ ub_env_enum(const char *last)
 	if (!syscall(API_ENV_ENUM, NULL, (uint32_t)last, (uint32_t)&env))
 		return (NULL);
 
-	if (env == NULL)
+	if (env == NULL || last == env)
 		/* no more env. variables to enumerate */
 		return (NULL);