certs: simplify $(srctree)/ handling and remove config_filename macro

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>