feat(shields): add helix shield

PR: #429

app/boards/shields/helix/Kconfig.defconfig

@@ -0,0 +1,29 @@
+# Copyright (c) 2020 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+if SHIELD_HELIX_LEFT
+
+config ZMK_KEYBOARD_NAME
+    default "Helix Left"
+
+config ZMK_SPLIT_BLE_ROLE_CENTRAL
+    default y
+
+endif
+
+if SHIELD_HELIX_RIGHT
+
+config ZMK_KEYBOARD_NAME
+    default "Helix Right"
+
+config USB
+    default y
+
+endif
+
+if SHIELD_HELIX_LEFT || SHIELD_HELIX_RIGHT
+
+config ZMK_SPLIT
+    default y
+
+endif