powerpc/boot: Fix build failures with -j 1

In commit 5e9dcb61 ("powerpc/boot: Expose Kconfig symbols to
wrapper") we added a dependency to serial.c on autoconf.h:

  $(obj)/serial.c: $(obj)/autoconf.h

This works when building in-tree (ie. with KBUILD_OUTPUT unset)
because the obj tree is the src tree.

But when building with eg. O=build and -j 1 the build fails:

  gcc ... -I../arch/powerpc/boot -c -o arch/powerpc/boot/serial.o arch/powerpc/boot/serial.c
  gcc: error: arch/powerpc/boot/serial.c: No such file or directory

Why this is only happening with -j 1 is not clear, when building with
-j greater than 1 somehow we decide to look for serial.c in the src
tree (../), eg:

  gcc -I../arch/powerpc/boot -c -o arch/powerpc/boot/serial.o ../arch/powerpc/boot/serial.c

Regardless we shouldn't be specifying a dependency on serial.c in the
build tree, we want to add a dependency to the version in $(srctree)
so fix the rule to say that.

Fixes: 5e9dcb61

 ("powerpc/boot: Expose Kconfig symbols to wrapper")
Tested-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>