Makefile: Strip leading spaces when preprocessing generated_defconfig

Makefile: Strip leading spaces when preprocessing generated_defconfig

Clang's preprocessor may emit extra spaces for lines starting with '#'.
Lines with these extra characters cannot be handled by Kconfig and will
be ignored with warnings like,

        unexpected data:  # CONFIG_OF_BOARD_FIXUP is not set

Those options that is expected to be assigned explicitly with N will be
set to the default value, messing up board configurations.

Let's sed these spaces away to ensure board configurations could be
correctly generated with Clang.

Link: https://github.com/llvm/llvm-project/issues/78778
Fixes: 2027e99e61a ("Makefile: Run defconfig files through the C preprocessor")
Reported-by: Nathaniel Hourt <I@nathaniel.land>
Signed-off-by: Yao Zi <ziyao@disroot.org>