pw_kvs: Introduce alignment to the entry header

- Add an 8-bit alignment field to the entry header. This makes the
  entry header a complete description of the entry's representation in
  flash (assuming the checksum algorithm is implied by the magic).
- Write basic tests for the EntryHeader class.

Change-Id: I7e48d5c16451ceb81f7c191d2f261e0a9bef14b7
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index cb9464b..9f267b0 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -81,6 +81,16 @@
 )
 
 pw_cc_test(
+    name = "entry_test",
+    srcs = [
+        "entry_test.cc",
+    ],
+    deps = [
+        ":pw_kvs",
+    ],
+)
+
+pw_cc_test(
     name = "key_value_store_test",
     srcs = ["key_value_store_test.cc"],
     deps = [