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;