Update Fuchsia SDK instructions to include netstack

Current version of Fuchsia SDK doesn't include netstack, which will be
required to run many tests. Updated the SDK instructions to include the
following packages in the SDK:
 - netstack: netstack service.
 - runtime-config: bootstrap configs to run netstack.

I'll update test_runner.py to start bootstrap and netstack after the
SDK is updated with these packages.

Bug: 731302
Change-Id: Ida1870f2135a878ad9e036ac4e1858fc7039d1c4
Reviewed-on: https://chromium-review.googlesource.com/581947
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488720}

docs/fuchsia_sdk_updates.md

@@ -29,14 +29,14 @@ Now verify that the build is stable:
 
         $ fboot
 
-0. Run tests to verify the build. All the tests should pass! If they don't then find out which of the test fixtures is failing and ping #cr-fuchsia on IRC to determine if it's a known issue. 
+0. Run tests to verify the build. All the tests should pass! If they don't then find out which of the test fixtures is failing and ping #cr-fuchsia on IRC to determine if it's a known issue.
 
         $ runtests
 
 ## Build and upload the SDK
 1. Configure and build a release mode build with sdk config:
 
-        $ ./packages/gn/gen.py --release --goma -m runtime
+        $ ./packages/gn/gen.py --release --goma -m runtime,runtime_config,netstack
         $ ./packages/gn/build.py --release -j1000
 
 0. Package the sysroot and tools.