Makefile: share $(DATA) on internal/external, keep record of build flags

This means you could make precompile, then make INTERNAL=on then make
INTERNAL=off and each time it would re-generate the go files only since
the flags have changed.

internal.go

@@ -14,7 +14,7 @@ const binaryType = "internal"
 
 func BuildTagsExternal() {}
 
-//go:embed data data/html data/web data/web/css data/web/js
+//go:embed build/data build/data/html build/data/web build/data/web/css build/data/web/js
 var loFS embed.FS
 
 //go:embed lang/common lang/admin lang/email lang/form lang/setup lang/pwreset lang/telegram
@@ -38,6 +38,6 @@ func FSJoin(elem ...string) string {
 
 func loadFilesystems(rootDir string, logger *logger.Logger) {
 	langFS = rewriteFS{laFS, "lang/"}
-	localFS = rewriteFS{loFS, "data/"}
+	localFS = rewriteFS{loFS, "build/data/"}
 	logger.Println("Using internal storage")
 }