toolchain: add a version that can be bumped to force toolchain/target rebuild

This can be used to ensure that the compiled code is up to date, when
something important changes in the toolchain.
A recent example of this is the gcc 11 fix for a code miscompilation issue

Signed-off-by: Felix Fietkau <nbd@nbd.name>

scripts/check-toolchain-clean.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+eval `grep CONFIG_GCC_VERSION .config`
+CONFIG_TOOLCHAIN_BUILD_VER="$CONFIG_GCC_VERSION-$(cat toolchain/build_version)"
+touch .toolchain_build_ver
+[ "$CONFIG_TOOLCHAIN_BUILD_VER" = "$(cat .toolchain_build_ver)" ] && exit 0
+echo "Toolchain build version changed, running make targetclean"
+make targetclean
+echo "$CONFIG_TOOLCHAIN_BUILD_VER" > .toolchain_build_ver
+exit 0