fix booting without hdmi plugged in

hardware/configuration-rpi-3b.nix

@@ -1,4 +1,4 @@
-{ lib, pkgs, ... }:
+{ lib, pkgs, config, ... }:
 {
   boot = {
     kernelPackages = pkgs.linuxPackages_latest;
@@ -7,4 +7,68 @@
 
   networking.useDHCP = false;
   networking.interfaces.eth0.useDHCP = true;
+
+  sdImage = lib.mkForce {
+    populateFirmwareCommands = let
+      configTxt = pkgs.writeText "config.txt" ''
+        [pi3]
+        kernel=u-boot-rpi3.bin
+        hdmi_force_hotplug=1
+
+        [pi02]
+        kernel=u-boot-rpi3.bin
+
+        [pi4]
+        kernel=u-boot-rpi4.bin
+        enable_gic=1
+        armstub=armstub8-gic.bin
+
+        # Otherwise the resolution will be weird in most cases, compared to
+        # what the pi3 firmware does by default.
+        disable_overscan=1
+
+        # Supported in newer board revisions
+        arm_boost=1
+
+        [cm4]
+        # Enable host mode on the 2711 built-in XHCI USB controller.
+        # This line should be removed if the legacy DWC2 controller is required
+        # (e.g. for USB device mode) or if USB support is not required.
+        otg_mode=1
+
+        [all]
+        # Boot in 64-bit mode.
+        arm_64bit=1
+
+        # U-Boot needs this to work, regardless of whether UART is actually used or not.
+        # Look in arch/arm/mach-bcm283x/Kconfig in the U-Boot tree to see if this is still
+        # a requirement in the future.
+        enable_uart=1
+
+        # Prevent the firmware from smashing the framebuffer setup done by the mainline kernel
+        # when attempting to show low-voltage or overtemperature warnings.
+        avoid_warnings=1
+      '';
+      in ''
+        (cd ${pkgs.raspberrypifw}/share/raspberrypi/boot && cp bootcode.bin fixup*.dat start*.elf $NIX_BUILD_TOP/firmware/)
+
+        # Add the config
+        cp ${configTxt} firmware/config.txt
+
+        # Add pi3 specific files
+        cp ${pkgs.ubootRaspberryPi3_64bit}/u-boot.bin firmware/u-boot-rpi3.bin
+
+        # Add pi4 specific files
+        cp ${pkgs.ubootRaspberryPi4_64bit}/u-boot.bin firmware/u-boot-rpi4.bin
+        cp ${pkgs.raspberrypi-armstubs}/armstub8-gic.bin firmware/armstub8-gic.bin
+        cp ${pkgs.raspberrypifw}/share/raspberrypi/boot/bcm2711-rpi-4-b.dtb firmware/
+        cp ${pkgs.raspberrypifw}/share/raspberrypi/boot/bcm2711-rpi-400.dtb firmware/
+        cp ${pkgs.raspberrypifw}/share/raspberrypi/boot/bcm2711-rpi-cm4.dtb firmware/
+        cp ${pkgs.raspberrypifw}/share/raspberrypi/boot/bcm2711-rpi-cm4s.dtb firmware/
+      '';
+    populateRootCommands = ''
+      mkdir -p ./files/boot
+      ${config.boot.loader.generic-extlinux-compatible.populateCmd} -c ${config.system.build.toplevel} -d ./files/boot
+    '';
+  };
 }