setup: start sketching out some bits

Tiny sketch of what we'd like to do. We'll be having an entrypoint,
modifications we can apply to directory structures and some primitives
to support those modifications.

Signed-off-by: Simon de Vlieger <supakeen@redhat.com>

cmd/posadka/main.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"flag"
+)
+
+func init() {
+}
+
+func main() {
+	flag.Parse()
+}

modify.go

@@ -0,0 +1,44 @@
+package posadka
+
+import (
+	"path/filepath"
+)
+
+func EmptyRootPassword(prefix string) error {
+	path := filepath.Join(prefix, "etc/passwd")
+
+	return ReplaceInFile(
+		path,
+		[]byte("root:x:"),
+		[]byte("root::"),
+	)
+}
+
+func EnableSystemRequest(prefix string) error {
+	//579 # Enable System Request debugging of the kernel
+	//580 if [ "$SYSRQ" != "" ]; then
+	//581     echo "= Enabling System Request debugging of the kernel."
+	//582     cat >> ${PREFIX}/etc/sysctl.d/arm-image-installer-sysrq.conf <<-EOH
+	//583         # Controls the System Request debugging functionality of the kernel
+	//584         kernel.sysrq = 1
+	//585     EOH
+	//586 fi
+	return nil
+}
+
+func ShowBoot(prefix string) error {
+	path := filepath.Join(prefix, "etc/passwd")
+
+	//# remove quiet from kargs
+	//if [ "$SHOWBOOT" != "" ]; then
+	//		 sed -i 's|rhgb quiet ||g' /tmp/boot/loader/entries/*.conf
+	//fi
+
+	return ReplaceInFile(
+		path,
+		[]byte("rhgb quiet "),
+		[]byte(""),
+	)
+
+	return nil
+}

primitive.go

@@ -0,0 +1,31 @@
+package posadka
+
+import (
+	"bytes"
+	"os"
+)
+
+// Replace `a` with `b` in a given file at `path`. This function is non-atomic
+// so be careful with crashing.
+func ReplaceInFile(path string, a, b []byte) error {
+	data, err := os.ReadFile(path)
+
+	if err != nil {
+		return err
+	}
+
+	data = bytes.Replace(
+		data,
+		a,
+		b,
+		1,
+	)
+
+	err = os.WriteFile(path, data, 0644)
+
+	if err != nil {
+		return err
+	}
+
+	return nil
+}