configs: use syntax CONFIG_FOO=n in tools-only_defconfig

The tools-only defconfig causes troubles on MacOSX due to the default
C compiler being Clang (LLVM) rather than GCC and more specifically
due to [1]. Therefore replace "$ CONFIG_FOO is not set" with the
equivalent "CONFIG_FOO=n" using the following command:

 $ sed -i -e 's/# \(CONFIG_[^ ]*\) is not set/\1=n/' \
       configs/tools-only_defconfig

This fixes the tools_only_macOS CI job on GitHub [2].

[1] https://github.com/llvm/llvm-project/issues/78778
[2] https://dev.azure.com/u-boot/u-boot/_build/results?buildId=9105&view=results



Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>