uboot-envtools: fix parallel building

uboot-envtools: fix parallel building

Recent envtools update to 2023.07.02 has introduced a breakage when trying
to parallel build with the following error:
/bin/sh: line 1: scripts/basic/fixdep: No such file or directory

Luckily it can easily be reproduced locally via a simple script so it was
not hard to bisect it down to upstream commit [1].

However, its not that commits fault, it just uncovered an issue with the
way we have been building envtools for a long time, maybe even from the
package introduction.

The issue is that we are trying to build envtools as one of the U-Boot
no-dot-config-targets but envtools was newer a valid target for it but
since we were creating the config headers that were not actually used it
was actually building all this time.

Since the blamed commit [1] a tool called printinitialenv is built and
now a proper config is actually required in order for prerequisites to
get built properly.

So, in order to properly fix this (Hopefully for good) lets stop pretending
that envtools are a valid no-dot-config-targets target and use the
tools-only defconfig which is meant exactly for just building the tools.
This will make a minimal config for the U-Boot sandbox target and then
envtools will build just fine in parallel mode (I tested with 32 threads).
We do hovewer need to override the ARCH passed by OpenWrt and set it to
sandbox as otherwise U-Boot will not find the required headers because the
ARCH is being overriden to an incorrect one.

[1] https://source.denx.de/u-boot/u-boot/-/commit/40b77f2a3ac13a7547c1b7c9c51a4090869de8f4

Fixes: 9db033005210 ("uboot-envtools: update to 2023.07.02")
Signed-off-by: Robert Marko <robert.marko@sartura.hr>