Implement directed vendor snapshot

Vendors can now generate only needed modules by setting the following
Makefile variables:

- DIRECTED_VENDOR_SNAPSHOT: set to true
- VENDOR_SNAPSHOT_MODULES: list of snapshot candidates

e.g.

DIRECTED_VENDOR_SNAPSHOT := true
VENDOR_SNAPSHOT_MODULES := toybox_vendor sh_vendor libbase libcutils ...

Bug: 157967325
Test: m dist vendor-snapshot after setting those in BoardConfig.mk
Change-Id: I6515a43d9358d94483d7c7fa2b066f9dd457f6aa

android/variable.go

@@ -309,6 +309,9 @@ type productVariables struct {
 	VndkUseCoreVariant         *bool `json:",omitempty"`
 	VndkSnapshotBuildArtifacts *bool `json:",omitempty"`
 
+	DirectedVendorSnapshot bool            `json:",omitempty"`
+	VendorSnapshotModules  map[string]bool `json:",omitempty"`
+
 	BoardVendorSepolicyDirs      []string `json:",omitempty"`
 	BoardOdmSepolicyDirs         []string `json:",omitempty"`
 	BoardReqdMaskPolicy          []string `json:",omitempty"`