Currently, the Kbuild core manipulates header search paths in a crazy way
[1].

To fix this mess, I want all Makefiles to add explicit $(srctree)/ to the
search paths in the srctree. Some Makefiles are already written in that
way, but not all. The goal of this work is to make the notation consistent,
and finally get rid of the gross hacks.

Having whitespaces after -I does not matter since commit 48f6e3cf
("kbuild: do not drop -I without parameter").

[1]: https://patchwork.kernel.org/patch/9632347/Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>