Use a property to enable the eth0 LAN port on the emulator

Change-Id: I26f4ec7fac9b7a8ad3013914967cf0f8fdec91ff
diff --git a/target/board/generic_x86/buildspec-generic_x86.mk b/target/board/generic_x86/buildspec-generic_x86.mk
index 5c4e6a5..fbc3947 100644
--- a/target/board/generic_x86/buildspec-generic_x86.mk
+++ b/target/board/generic_x86/buildspec-generic_x86.mk
@@ -1,3 +1,7 @@
 BUILD_ENV_SEQUENCE_NUMBER := 9
 DISABLE_DEXPREOPT := true
 TARGET_ARCH := x86
+
+# The eth0 device should be started with dhcp on boot.
+# Useful for emulators that don't provide a wifi connection.
+NET_ETH0_STARTONBOOT := true