[NETD-BPF#34] Add a tag for bpf to specify the install folder

Currently, the bpf module netd.o (source system/netd/bpf_progs/netd.c)
will be built to /system/etc/bpf/netd.o. In Android T, it will be moved
to mainline module com.android.tethering.

The expected behavior is:
- In T device, it uses the netd.o in mainline module.
- In pre-T devices, it uses the original netd, built from platform.

However, netd.o will be double loaded if the tethering module is
installed in Pre-T devices. Because:
1. bpf in apex is packed into /apex/MAINLINE_MODULE/etc/bpf/
2. bpf in platform is packed into /system/etc/bpf/
3. bpfloader in pre-T loads ANY bpf modules under
/apex/com.android.tethering/etc/bpf/ and /system/etc/bpf/.

We can't change the behavior of bpfloader in pre-T devices. We can't
delete the /system/etc/bpf/netd.o from pre-T devices. Both of them are
not mainline modules. So the mainlined netd.o needs to be packed into a
folder other than /apex/com.android.tethering/etc/bpf/ or
/system/etc/bpf/.

This commit adds a tag 'sub_dir' for bpf module. The installation path
of bpf modules will be:
- /system/etc/bpf/SUB_DIR/     (for platform code)
- /apex/MAINLINE_MODULE/etc/bpf/SUB_DIR/     (for mainline module)

Bug: 202086915
Test: add test in apex_test.go and build
Change-Id: Icc6619768ab006de9f86620a7df1bb2853eaba13

bpf/bpf.go

@@ -54,12 +54,16 @@ type BpfModule interface {
 	android.Module
 
 	OutputFiles(tag string) (android.Paths, error)
+
+	// Returns the sub install directory if the bpf module is included by apex.
+	SubDir() string
 }
 
 type BpfProperties struct {
 	Srcs         []string `android:"path"`
 	Cflags       []string
 	Include_dirs []string
+	Sub_dir      string
 }
 
 type bpf struct {
@@ -121,6 +125,10 @@ func (bpf *bpf) AndroidMk() android.AndroidMkData {
 			fmt.Fprintln(w)
 			fmt.Fprintln(w, "LOCAL_PATH :=", moduleDir)
 			fmt.Fprintln(w)
+			localModulePath := "LOCAL_MODULE_PATH := $(TARGET_OUT_ETC)/bpf"
+			if len(bpf.properties.Sub_dir) > 0 {
+				localModulePath += "/" + bpf.properties.Sub_dir
+			}
 			for _, obj := range bpf.objs {
 				objName := name + "_" + obj.Base()
 				names = append(names, objName)
@@ -130,7 +138,7 @@ func (bpf *bpf) AndroidMk() android.AndroidMkData {
 				fmt.Fprintln(w, "LOCAL_PREBUILT_MODULE_FILE :=", obj.String())
 				fmt.Fprintln(w, "LOCAL_MODULE_STEM :=", obj.Base())
 				fmt.Fprintln(w, "LOCAL_MODULE_CLASS := ETC")
-				fmt.Fprintln(w, "LOCAL_MODULE_PATH := $(TARGET_OUT_ETC)/bpf")
+				fmt.Fprintln(w, localModulePath)
 				fmt.Fprintln(w, "include $(BUILD_PREBUILT)")
 				fmt.Fprintln(w)
 			}
@@ -154,6 +162,10 @@ func (bpf *bpf) OutputFiles(tag string) (android.Paths, error) {
 	}
 }
 
+func (bpf *bpf) SubDir() string {
+	return bpf.properties.Sub_dir
+}
+
 var _ android.OutputFileProducer = (*bpf)(nil)
 
 func BpfFactory() android.Module {