The complex macro, config_filename, was introduced to do:
[1] drop double-quotes from the string value
[2] add $(srctree)/ prefix in case the file is not found in $(objtree)
[3] escape spaces and more
[1] will be more generally handled by Kconfig later.
As for [2], Kbuild uses VPATH to search for files in $(objtree),
$(srctree) in this order. GNU Make can natively handle it.
As for [3], converting $(space) to $(space_escape) back and forth looks
questionable to me. It is well-known that GNU Make cannot handle file
paths with spaces in the first place.
Instead of using the complex macro, use $< so it will be expanded to
the file path of the key.
Remove config_filename, finally.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
