Add firmware/seabios dir in the cros-chrome-sdk to cros VM test data deps.

Needed to pack up the bios-256k.bin file in the test isolates. The latest lkgm
bumps fail w/o it:
https://chromium-swarm.appspot.com/task?id=40e0280b72f15510

NOTRY=true

Bug: 897540
Change-Id: If731c73a677cff02523b17599a1a50629b6ff430
Reviewed-on: https://chromium-review.googlesource.com/c/1308716
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604096}

build/config/chromeos/rules.gni

@@ -34,6 +34,7 @@ template("generate_vm_runner_script") {
       "//build/cros_cache/chrome-sdk/tarballs/${cros_board}+${cros_sdk_version}"
   _vm_image_path = "${_cache_path_prefix}+chromiumos_qemu_image.tar.xz/"
   _qemu_dir = "${_cache_path_prefix}+app-emulation/"
+  _firmware_dir = "${_cache_path_prefix}+sys-firmware/"
 
   forward_variables_from(invoker,
                          [
@@ -91,6 +92,7 @@ template("generate_vm_runner_script") {
       # as data here so that changes to it will trigger analyze.
       "//chromeos/CHROMEOS_LKGM",
       "//third_party/chromite/",
+      _firmware_dir,
       _vm_image_path,
       _qemu_dir,
     ]