check-toolchain-clean.sh: workaround stray rebuilds

It seems, that there are currently some unhandled corner cases in which
`.toolchain_build_ver` results in empty file and thus forcing rebuilds,
even if the toolchain was build correctly just a few moments ago. Until
proper fix is found, workaround that by checking for this corner case
and simply populate `.toolchain_build_ver` file.

While at it, improve the UX and display version mismatch, so it's more
clear what has forced the rebuild:

 "Toolchain build version changed (11.2.0-1 != ), running make targetclean"

References: https://gitlab.com/ynezz/openwrt/-/jobs/2133332533/raw
Signed-off-by: Petr Štetiar <ynezz@true.cz>

scripts/check-toolchain-clean.sh

@@ -2,8 +2,13 @@
 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" = "$(cat .toolchain_build_ver)" ] && exit 0
-echo "Toolchain build version changed, running make targetclean"
+CURRENT_TOOLCHAIN_BUILD_VER="$(cat .toolchain_build_ver)"
+[ -z "$CURRENT_TOOLCHAIN_BUILD_VER" ] && {
+	echo "$CONFIG_TOOLCHAIN_BUILD_VER" > .toolchain_build_ver
+	exit 0
+}
+[ "$CONFIG_TOOLCHAIN_BUILD_VER" = "$CURRENT_TOOLCHAIN_BUILD_VER" ] && exit 0
+echo "Toolchain build version changed ($CONFIG_TOOLCHAIN_BUILD_VER != $CURRENT_TOOLCHAIN_BUILD_VER), running make targetclean"
 make targetclean
 echo "$CONFIG_TOOLCHAIN_BUILD_VER" > .toolchain_build_ver
 exit 0