kbuild: use pattern rule for building built-in.a in sub-directories

The built-in.a in a sub-directory is created by descending into that
directory. It does not depend on the other sub-directories. Loosen
the dependency.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.build

@@ -372,7 +372,7 @@ $(obj)/%.asn1.c $(obj)/%.asn1.h: $(src)/%.asn1 $(objtree)/scripts/asn1_compiler
 # ---------------------------------------------------------------------------
 
 # To build objects in subdirs, we need to descend into the directories
-$(sort $(subdir-obj-y)): $(subdir-ym) ;
+$(obj)/%/built-in.a: $(obj)/% ;
 
 #
 # Rule to compile a set of .o files into one .a file (without symbol table)