chromeos: Only declare cros_vm_launcher target when the board is set.

Building "all" for linux-chromeos is currently broken due to this target
getting built despite no board being set.

Bug: None
Test: built "all" w/ linux-chromeos

Change-Id: I79383869d3617269753db461b99068f73f7f0cfb
Reviewed-on: https://chromium-review.googlesource.com/1045107
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Ben Pastene <[email protected]>
Cr-Commit-Position: refs/heads/master@{#556252}
diff --git a/chromeos/BUILD.gn b/chromeos/BUILD.gn
index 4f66dca..194e160 100644
--- a/chromeos/BUILD.gn
+++ b/chromeos/BUILD.gn
@@ -788,8 +788,10 @@
   ]
 }
 
-generate_vm_runner_script("cros_vm_launcher") {
-  generated_script = "$root_build_dir/bin/launch_cros_vm"
+if (cros_board != "") {
+  generate_vm_runner_script("cros_vm_launcher") {
+    generated_script = "$root_build_dir/bin/launch_cros_vm"
+  }
 }
 
 fuzzer_test("variable_expander_fuzzer") {