qemu: buildroot: fix networking

Adds the missing startup file to the buildroot-generated filesystem so
that networking is available in the VM as claimed by docs/qemu.md.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/common.mk b/common.mk
index 39d2878..3abdf4d 100644
--- a/common.mk
+++ b/common.mk
@@ -201,6 +201,8 @@
 	@touch ../out-br/extra.conf
 	@echo "BR2_TARGET_GENERIC_GETTY_PORT=\"$(BUILDROOT_GETTY_PORT)\"" >> \
 		../out-br/extra.conf
+	@echo "BR2_ROOTFS_OVERLAY=\"$(ROOT)/build/br-ext/board/qemu/overlay\"" >> \
+		../out-br/extra.conf
 	@echo "BR2_PACKAGE_OPTEE_TEST_CROSS_COMPILE=\"$(CROSS_COMPILE_S_USER)\"" >> \
 		../out-br/extra.conf
 	@echo "BR2_PACKAGE_OPTEE_EXAMPLES_CROSS_COMPILE=\"$(CROSS_COMPILE_S_USER)\"" >> \