kbuild: Fix signing issue for external modules

kbuild: Fix signing issue for external modules

[ Upstream commit 25ff08aa43e373a61c3e36fc7d7cae88ed0fc2d7 ]

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: 13b25489b6f8 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>