The postlink Makefile must include include/config/auto.conf to get the
kernel configuration variables. But in a clean kernel directory this
file does not exist, causing make to bail with the error:

arch/mips/Makefile.postlink:10: include/config/auto.conf: No such file or directory
make[1]: *** No rule to make target 'include/config/auto.conf'.  Stop.
Makefile:1290: recipe for target 'vmlinuxclean' failed

Fix this by using "-include" to not cause a Make error when the file
does not exist.

Fixes: 44079d3509ae ("MIPS: Use Makefile.postlink to insert relocations into vmlinux")
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15136/
Signed-off-by: James Hogan <james.hogan@imgtec.com>

When relocatable support for MIPS was merged, there was no support for
an architecture to add a postlink step for vmlinux. This meant that only
invoking a target within the boot directory, such as uImage, caused the
relocations to be inserted into vmlinux. Building just the vmlinux
target would result in a relocatable kernel with no relocation
information present.

Commit fbe6e37dab97 ("kbuild: add arch specific post-link Makefile")
recified this situation, so MIPS can now define a postlink step to add
relocation information into vmlinux, and remove the additional steps
tacked onto boot targets.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Tested-by: Steven J. Hill <steven.hill@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14554/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>