Implement support for build_prop.Footer_files

Test: m system-build.prop && tail ${OUT}/system/build.prop
Bug: 378710882
Change-Id: Ia1dd71cbcb62fbe6453ed6c48409cd1f42879986
diff --git a/android/build_prop.go b/android/build_prop.go
index cda56f1..2f71bc0 100644
--- a/android/build_prop.go
+++ b/android/build_prop.go
@@ -15,6 +15,8 @@
 package android
 
 import (
+	"fmt"
+
 	"github.com/google/blueprint/proptools"
 )
 
@@ -173,7 +175,16 @@
 	postProcessCmd.Text(outputFilePath.String())
 	postProcessCmd.Flags(p.properties.Block_list)
 
-	rule.Command().Text("echo").Text(proptools.NinjaAndShellEscape("# end of file")).FlagWithArg(">> ", outputFilePath.String())
+	for _, footer := range p.properties.Footer_files {
+		path := PathForModuleSrc(ctx, footer)
+		rule.appendText(outputFilePath, "####################################")
+		rule.appendTextf(outputFilePath, "# Adding footer from %v", footer)
+		rule.appendTextf(outputFilePath, "# with path %v", path)
+		rule.appendText(outputFilePath, "####################################")
+		rule.Command().Text("cat").FlagWithInput("", path).FlagWithArg(">> ", outputFilePath.String())
+	}
+
+	rule.appendText(outputFilePath, "# end of file")
 
 	rule.Build(ctx.ModuleName(), "generating build.prop")
 
@@ -184,6 +195,14 @@
 	p.outputFilePath = outputFilePath
 }
 
+func (r *RuleBuilder) appendText(path ModuleOutPath, text string) {
+	r.Command().Text("echo").Text(proptools.NinjaAndShellEscape(text)).FlagWithArg(">> ", path.String())
+}
+
+func (r *RuleBuilder) appendTextf(path ModuleOutPath, format string, a ...any) {
+	r.appendText(path, fmt.Sprintf(format, a...))
+}
+
 func (p *buildPropModule) AndroidMkEntries() []AndroidMkEntries {
 	return []AndroidMkEntries{{
 		Class:      "ETC",