add evfbo (evdev + fbdev + stdout) backend

During a normal boot in the postmarketOS initramfs, the TTY is not
easily accessible. The on-screen keyboard is expected to send the
password to stdout instead of to a uinput device. Add the evfbo backend
to support full-disk encryption.

src/stdio/mod.rs

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-3.0-only
+/*
+ * Copyright (c) 2025, Richard Acayan. All rights reserved.
+ */
+
+mod output;
+
+pub use self::output::Output;

src/stdio/output.rs

@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: GPL-3.0-only
+/*
+ * Copyright (c) 2025, Richard Acayan. All rights reserved.
+ */
+
+use crate::core::Keyboard;
+use crate::core::Layout;
+use std::iter::FromIterator;
+use xkeysym::Keysym;
+
+pub struct Output {
+    back: String,
+    front: String,
+    complete: bool,
+}
+
+impl Output {
+    pub fn new() -> Self
+    {
+        Self {
+            back: String::new(),
+            front: String::new(),
+            complete: false,
+        }
+    }
+
+    #[inline(always)]
+    pub fn complete(&self) -> bool
+    {
+        self.complete
+    }
+}
+
+impl Keyboard for Output {
+    fn key_supported(&self, sym: Keysym) -> bool
+    {
+        match sym {
+            Keysym::Return => true,
+            Keysym::BackSpace => true,
+            Keysym::Delete => true,
+            Keysym::Left => true,
+            Keysym::Right => true,
+            _ => false,
+        }
+    }
+
+    fn press(&mut self, sym: Keysym)
+    {
+        match sym {
+            Keysym::Return => {
+                let front = self.front.chars().rev();
+                let front = String::from_iter(front);
+
+                println!("{}{}", self.back, front);
+
+                self.complete = true;
+            },
+            Keysym::BackSpace => {
+                self.back.pop();
+            },
+            Keysym::Delete => {
+                self.front.pop();
+            },
+            Keysym::Left => {
+                if let Some(c) = self.back.pop() {
+                    self.front.push(c);
+                }
+            },
+            Keysym::Right => {
+                if let Some(c) = self.front.pop() {
+                    self.back.push(c);
+                }
+            },
+            _ => (),
+        }
+    }
+
+    fn release(&mut self, _: Keysym)
+    {
+    }
+
+    fn text(&mut self, text: &str)
+    {
+        for c in text.chars() {
+            self.back.push(c);
+        }
+    }
+
+    fn change_layout(&mut self, _: &Layout)
+    {
+    }
+}