nixos/sd-image-armv7l-multiplatform: Port RPi config.txt changes from aarch64 image

As was done in commit cd2e740dde9541ad5f1d9efd93bcb5a967379ece.

Changed files
+7
nixos
modules
+7
nixos/modules/installer/cd-dvd/sd-image-armv7l-multiplatform.nix
···
sdImage = {
populateBootCommands = let
configTxt = pkgs.writeText "config.txt" ''
+
# 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
+
[pi2]
kernel=u-boot-rpi2.bin
[pi3]
kernel=u-boot-rpi3.bin
+
+
# U-Boot used to need this to work, regardless of whether UART is actually used or not.
+
# TODO: check when/if this can be removed.
enable_uart=1
'';
in ''