Start using the toybox prebuilt

Prepend the toybox symlink directory to $PATH, and stop generating
symlinks into out/.path for tools used from toybox.

We don't (yet?) have toybox prebuilts for darwin, so apply the above only to Linux, and preserve existing behavior on Darwin.

Test: check the build banner for uname results
Test: m
Change-Id: I37fc380381e65a628cdc131d462fd4441eacfe9d
diff --git a/ui/build/path.go b/ui/build/path.go
index 52658ef..8260ff9 100644
--- a/ui/build/path.go
+++ b/ui/build/path.go
@@ -19,6 +19,7 @@
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 
 	"github.com/google/blueprint/microfactory"
@@ -144,6 +145,13 @@
 	}
 
 	myPath, _ = filepath.Abs(myPath)
+
+	// Use the toybox prebuilts on linux
+	if runtime.GOOS == "linux" {
+		toyboxPath, _ := filepath.Abs("prebuilts/build-tools/toybox/linux-x86")
+		myPath = toyboxPath + string(os.PathListSeparator) + myPath
+	}
+
 	config.Environment().Set("PATH", myPath)
 	config.pathReplaced = true
 }