kbuild: replace sed with $(subst ) or $(patsubst )

For simple text replacement, it is better to use a built-in function
instead of sed if possible. You can save one process forking.

I do not mean to replace all sed invocations because GNU Make itself
does not support regular expression (unless you use guile).

I just replaced simple ones.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>