kbuild: rebuild .vmlinux.export.o when its prerequisite is updated

When include/linux/export-internal.h is updated, .vmlinux.export.o
must be rebuilt, but it does not happen because its rule is hidden
behind scripts/link-vmlinux.sh.

Move it out of the shell script, so that Make can see the dependency
between vmlinux and .vmlinux.export.o.

Move the vmlinux rule to scripts/Makefile.vmlinux.

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

scripts/link-vmlinux.sh

@@ -199,7 +199,6 @@ cleanup()
 	rm -f System.map
 	rm -f vmlinux
 	rm -f vmlinux.map
-	rm -f .vmlinux.export.c
 }
 
 # Use "make V=1" to debug this script
@@ -214,10 +213,6 @@ if [ "$1" = "clean" ]; then
 	exit 0
 fi
 
-if is_enabled CONFIG_MODULES; then
-	${MAKE} -f "${srctree}/scripts/Makefile.vmlinux" .vmlinux.export.o
-fi
-
 ${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init init/version-timestamp.o
 
 btf_vmlinux_bin_o=""