kbuild: fix inconsistent indentation in top Makefile

Commit 3b9ab248 ("kbuild: use 4-space indentation when followed
by conditionals") introduced inconsistent indentation because it
deliberately touched only the conditional directives to minimize the
change set.

This commit reformats some blocks in the top Makefile so they are
consistently indented with 4 spaces.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>