Trianglesphere/oom fix (ethereum#297)

* Adds UseLightweightKDF option to mobile config

The mobile geth config options were pared down tp reduce complexity, but
the UseLightweightKDF option was not added. Because we use Geth to store
our keys on mobile, we want to use the lightweight KDF otherwise we get
OOM crashes on certain phones.

* Sets node config KDF parameter correctly

It needed to be passed through from mobile config to node config.

* Adds links for reference

mobile/geth.go

@@ -90,6 +90,11 @@ type NodeConfig struct {
 	// This has to be integer since Enum exports to Java are not supported by "gomobile"
 	// See getSyncMode(syncMode int)
 	SyncMode int
+
+	// UseLightweightKDF lowers the memory and CPU requirements of the key store
+	// scrypt KDF at the expense of security.
+	// See https://geth.ethereum.org/doc/Mobile_Account-management for reference
+	UseLightweightKDF bool
 }
 
 // defaultNodeConfig contains the default node configuration values to use if all
@@ -132,11 +137,12 @@ func NewNode(datadir string, config *NodeConfig) (stack *Node, _ error) {
 
 	// Create the empty networking stack
 	nodeConf := &node.Config{
-		Name:        clientIdentifier,
-		Version:     params.VersionWithMeta,
-		DataDir:     datadir,
-		KeyStoreDir: filepath.Join(datadir, "keystore"), // Mobile should never use internal keystores!
-		IPCPath:     "geth.ipc",
+		Name:              clientIdentifier,
+		Version:           params.VersionWithMeta,
+		DataDir:           datadir,
+		KeyStoreDir:       filepath.Join(datadir, "keystore"), // Mobile should never use internal keystores!
+		UseLightweightKDF: config.UseLightweightKDF,
+		IPCPath:           "geth.ipc",
 		P2P: p2p.Config{
 			NoDiscovery:      true,
 			DiscoveryV5:      false,