androidmk: add BUILD_OS conditionals

Some Android.mk files use BUILD_OS instead of HOST_OS for selecting
options when building host tools.

Change-Id: Ic4869cdee01da050364bb21b47f3de8fd8adaa96
diff --git a/androidmk/cmd/androidmk/android.go b/androidmk/cmd/androidmk/android.go
index 71c5832..44165e8 100644
--- a/androidmk/cmd/androidmk/android.go
+++ b/androidmk/cmd/androidmk/android.go
@@ -93,6 +93,10 @@
 	"($(HOST_OS), windows)": {"target", "windows"},
 	"($(HOST_OS),linux)":    {"target", "linux"},
 	"($(HOST_OS), linux)":   {"target", "linux"},
+	"($(BUILD_OS),darwin)":  {"target", "darwin"},
+	"($(BUILD_OS), darwin)": {"target", "darwin"},
+	"($(BUILD_OS),linux)":   {"target", "linux"},
+	"($(BUILD_OS), linux)":  {"target", "linux"},
 }
 
 func mydir(args []string) string {