pw_kvs: FlashPartition Init function

This change adds an Init() function to the flash partition to perform
any initialization that cannot be done in the constructor (or when
static constructors are disabled).

Change-Id: Idd2418bcfdc0a2e6c93ea2115eed5d4e2268518f
diff --git a/pw_kvs/public/pw_kvs/flash_memory.h b/pw_kvs/public/pw_kvs/flash_memory.h
index 0ce3378..48a0d69 100644
--- a/pw_kvs/public/pw_kvs/flash_memory.h
+++ b/pw_kvs/public/pw_kvs/flash_memory.h
@@ -154,6 +154,9 @@
 
   virtual ~FlashPartition() = default;
 
+  // Performs any required partition or flash-level initialization.
+  virtual Status Init() { return Status::OK; }
+
   // Erase num_sectors starting at a given address. Blocking call.
   // Address should be on a sector boundary.
   // Returns: OK, on success.