Add pw_build_host_tools variable

This change adds a variable which indicates whether to build host-side
tools.

Change-Id: Ib3fa2291492bfc942d38e8488ee3b94bdcbc09f3
diff --git a/pw_vars_default.gni b/pw_vars_default.gni
index baec2b4..92512f3 100644
--- a/pw_vars_default.gni
+++ b/pw_vars_default.gni
@@ -19,6 +19,9 @@
 # to build for it. This is done in a .gni file specific to the target by first
 # importing this file, then changing the variables as desired.
 
+# Whether to build host-side tooling.
+pw_build_host_tools = true
+
 # Options which configure the executable targets created in Pigweed builds.
 pw_executable_config = {
   # The name of the GN target type used to build Pigweed executables.