kbuild: pkg: use -f $(srctree)/Makefile to recurse to top Makefile

'$(MAKE) KBUILD_SRC=' changes the working directory back and forth
between objtree and srctree.

It is better to recurse to the top-level Makefile directly.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>