pw_kvs: Apply config pattern; configure log level

- Add PW_KVS_LOG_LEVEL to the pw_kvs config file and update code to
  use it.
- Define a pw_kvs:config facade that be used to override configuration
  values.
- Set up the empty "$dir_pw_build:empty" target that all modules use as
  their default configuration backend via the
  pw_build_DEFAULT_MODULE_CONFIGURATION variable.

Change-Id: Idf54d93678ffbd65e9e02a10454df4479f73827a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/21240
Reviewed-by: David Rogers <davidrogers@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_kvs/key_value_store.cc b/pw_kvs/key_value_store.cc
index 752590d..534313e 100644
--- a/pw_kvs/key_value_store.cc
+++ b/pw_kvs/key_value_store.cc
@@ -13,6 +13,7 @@
 // the License.
 
 #define PW_LOG_MODULE_NAME "KVS"
+#define PW_LOG_LEVEL PW_KVS_LOG_LEVEL
 #define PW_LOG_USE_ULTRA_SHORT_NAMES 1
 
 #include "pw_kvs/key_value_store.h"
@@ -23,6 +24,7 @@
 #include <type_traits>
 
 #include "pw_assert/assert.h"
+#include "pw_kvs_private/config.h"
 #include "pw_log/log.h"
 #include "pw_status/try.h"