check-toolchain-clean.sh: fix shellcheck warnings

Fixes following complaints and suggestions:

 In scripts/check-toolchain-clean.sh line 2:
 eval `grep CONFIG_GCC_VERSION .config`
      ^-- SC2046 (warning): Quote this to prevent word splitting.
      ^-- SC2006 (style): Use $(...) notation instead of legacy backticks `...`.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

scripts/check-toolchain-clean.sh

 #!/bin/sh
-eval `grep CONFIG_GCC_VERSION .config`
+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