env: Complete generic support for writable list

This completes what 890feecaab started by selecting ENV_APPEND and
loading the default env before any other sources. This ensures that load
operations pick up all non-writable vars from the default env and only
permitted parts from other locations according to the regular
priorities.

With this change, boards only need to define the list of writable
variables but no longer have to provide a custom env_get_location
implementation.

CC: Joe Hershberger <joe.hershberger@ni.com>
CC: Marek Vasut <marex@denx.de>
CC: Stefan Herbrechtsmeier <stefan.herbrechtsmeier-oss@weidmueller.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Marek Vasut <marex@denx.de>

env/Kconfig

@@ -733,6 +733,7 @@ config ENV_APPEND
 
 config ENV_WRITEABLE_LIST
 	bool "Permit write access only to listed variables"
+	select ENV_APPEND
 	help
 	  If defined, only environment variables which explicitly set the 'w'
 	  writeable flag can be written and modified at runtime. No variables

env/env.c

@@ -192,6 +192,14 @@ int env_load(void)
 	int best_prio = -1;
 	int prio;
 
+	if (CONFIG_IS_ENABLED(ENV_WRITEABLE_LIST)) {
+		/*
+		 * When using a list of writeable variables, the baseline comes
+		 * from the built-in default env. So load this first.
+		 */
+		env_set_default(NULL, 0);
+	}
+
 	for (prio = 0; (drv = env_driver_lookup(ENVOP_LOAD, prio)); prio++) {
 		int ret;