bootconfig: Remove duplicate included header file linux/bootconfig.h

bootconfig: Remove duplicate included header file linux/bootconfig.h

The header file linux/bootconfig.h is included whether __KERNEL__ is
defined or not.

Include it only once before the #ifdef/#else/#endif preprocessor
directives and remove the following make includecheck warning:

  linux/bootconfig.h is included more than once

Move the comment to the top and delete the now empty #else block.

Link: https://lore.kernel.org/all/20240711084315.1507-1-thorsten.blum@toblux.com/
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>