Prefer "external booting" u-boot by default.

Less rude for end-users expecting to be able to SD boot once flashed.

README.md

@@ -97,6 +97,9 @@ If you rather USB and SD card is tried before the eMMC, `pkgs.uBootPinebookProEx
 can be installed, which has an alternative patch set added on top that will
 change the boot order.
 
+The SD image is built using the "alternative boot order" u-boot. Thus, flashing
+the image to your eMMC keeps external devices bootable.
+
 
 ## Keyboard firmware
 

configuration.nix

@@ -1,7 +1,7 @@
 { config, pkgs, lib, ... }:
 
 let
-  uboot = pkgs.uBootPinebookPro;
+  uboot = pkgs.uBootPinebookProExternalFirst;
 in
 {
   imports = [