kbuild: Fix signing issue for external modules

When running the sign script the kernel is within the source directory
of external modules. This caused issues when the kernel uses relative
paths, like:

make[5]: Entering directory '/build/client/devel/kernel/work/linux-2.6'
make[6]: Entering directory '/build/client/devel/addmodules/vtx/work/vtx'
   INSTALL /build/client/devel/addmodules/vtx/_/lib/modules/6.13.0-devel+/extra/vtx.ko
   SIGN    /build/client/devel/addmodules/vtx/_/lib/modules/6.13.0-devel+/extra/vtx.ko
/bin/sh: 1: scripts/sign-file: not found
   DEPMOD  /build/client/devel/addmodules/vtx/_/lib/modules/6.13.0-devel+

Working around it by using absolute pathes here.

Fixes: 13b25489 ("kbuild: change working directory to external module directory with M=")
Signed-off-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>