rewrite preinit as very small C program

By using the kernel "nolibc" header to avoid requiring a C library, we
can bring the initramfs size to around 4k

This does involve a tiny bit of inline mips assembly which I'm not
sure about. gcc seems unwilling to generate the code to load $gp at
function entry of main(), so we do it by hand - but I'd rather find
out why gcc doesn't.

pkgs/default.nix

@@ -40,6 +40,7 @@
   # https://sourceforge.net/p/squashfs/mailman/message/26599379/
   lzma = callPackage ./lzma {};
 
+  preinit = callPackage ./preinit {};
   swconfig = callPackage ./swconfig {};
 
   openwrt = callPackage ./openwrt {};