`ubootTools` build broke after https://github.com/NixOS/nixpkgs/pull/210004
where we started dropping default libc include path and switched to
`-idirafter` way of specifying libc headers.
Unfortunately the way it's implemented it injects -idirafter after
user's flags, not before. That allows users to inject their paths before
libc include paths, not after (as it would notmally happen).
The change works it around for u-boot by pulling -idirafter libc flags
before user's flags.
8b3a31f923