dragon: Use device/ makefiles to install supplicant_overlay.conf files

Currently, this is how the overlay files get built into the image:

device/google/dragon/device.mk says:
 $(call inherit-product-if-exists, vendor/google_devices/dragon/device-vendor.mk)

vendor/google_devices/dragon/device-vendor.mk says:
 $(call inherit-product-if-exists, vendor/google_devices/dragon/firmware/bcm4354/device-bcm.mk)

vendor/google_devices/dragon/firmware/bcm4354/device-bcm.mk includes this makefile:
 hardware/broadcom/wlan/bcmdhd/config/config-bcm.mk
   which installs:
     /system/etc/wifi/p2p_supplicant_overlay.conf
     /system/etc/wifi/wpa_supplicant_overlay.conf

On AOSP builds, the vendor/ tree is replaced by a minimal version that
only handles binary blobs.  Therefore, we'll want
device/google/dragon/device.mk to include the hardware/... makefile
directly, instead of through vendor/...

BUG=25870728
TEST=`rm -rf out` and then rebuild internal + AOSP trees

Change-Id: I1e495c1badb875625f63f799b72b5ca290c3a8e5
Signed-off-by: Kevin Cernekee <cernekee@google.com>
diff --git a/device.mk b/device.mk
index 6233f12..36b08af 100644
--- a/device.mk
+++ b/device.mk
@@ -194,6 +194,7 @@
 
 $(call inherit-product-if-exists, hardware/nvidia/tegra132/tegra132.mk)
 $(call inherit-product-if-exists, vendor/google_devices/dragon/device-vendor.mk)
+$(call inherit-product-if-exists, hardware/broadcom/wlan/bcmdhd/config/config-bcm.mk)
 
 ENABLE_LIBDRM := true
 BOARD_GPU_DRIVERS := tegra