dkms: error out if $arch cannot be determined

if for some reason $arch cannot be determined (e.g. due to a broken rpm
shim on distributions using a different package manager), dkms will
happily build a module, placing state info in
/var/lib/dkms/$module/$version/$kernel//, but that subsequently causes
errors because the module is not considered to be installed and at the
same time cannot be removed any more

Before:

sid# dkms status
sid# ln -s /bin/true /usr/bin/rpm
sid# dkms add dkms_test/1.0
Creating symlink /var/lib/dkms/dkms_test/1.0/source -> /usr/src/dkms_test-1.0
sid# dkms status
dkms_test/1.0: added
sid# dkms build -k 6.1.0-9-amd64 dkms_test/1.0
Sign command: /usr/lib/linux-kbuild-6.1/scripts/sign-file
Signing key: /var/lib/dkms/mok.key
Public certificate (MOK): /var/lib/dkms/mok.pub

Building module:
Cleaning build area...
make -j16 KERNELRELEASE=6.1.0-9-amd64 -C
/lib/modules/6.1.0-9-amd64/build M=/var/lib/dkms/dkms_test/1.0/build...
Signing module /var/lib/dkms/dkms_test/1.0/build/dkms_test.ko
Cleaning build area...
sid# dkms status
dkms_test/1.0: added
sid# dkms build -k 6.1.0-9-amd64 dkms_test/1.0
Sign command: /usr/lib/linux-kbuild-6.1/scripts/sign-file
Signing key: /var/lib/dkms/mok.key
Public certificate (MOK): /var/lib/dkms/mok.pub
Error! This module/version has already been built on: 6.1.0-9-amd64
Directory /var/lib/dkms/dkms_test/1.0/6.1.0-9-amd64/ already exists. Use
the dkms remove function before trying to build again.
sid# dkms status
dkms_test/1.0: added
sid# dkms remove -k 6.1.0-9-amd64 dkms_test/1.0
Module dkms_test 1.0 is not installed for kernel 6.1.0-9-amd64 ().  Skipping...
Module dkms_test 1.0 is not built for kernel 6.1.0-9-amd64 ().  Skipping...
sid# dkms status
dkms_test/1.0: added
sid# dkms remove dkms_test/1.0 --all
sid# dkms status
dkms_test/1.0: added
sid# find /var/lib/dkms/dkms_test/1.0/
/var/lib/dkms/dkms_test/1.0/
/var/lib/dkms/dkms_test/1.0/6.1.0-9-amd64
/var/lib/dkms/dkms_test/1.0/6.1.0-9-amd64/module
/var/lib/dkms/dkms_test/1.0/6.1.0-9-amd64/module/dkms_test.ko
/var/lib/dkms/dkms_test/1.0/6.1.0-9-amd64/log
/var/lib/dkms/dkms_test/1.0/6.1.0-9-amd64/log/make.log
/var/lib/dkms/dkms_test/1.0/6.1.0-9-amd64/log/.config
/var/lib/dkms/dkms_test/1.0/source

After:

sid# dkms status
sid# ln -s /bin/true /usr/bin/rpm
sid# dkms add dkms_test/1.0
Error! Could not determine architecture.
sid# echo $?
12

to manually recover from the bug use
  rm -rf /var/lib/dkms/$module/$version
and add/build/install the module(s) again

dkms.in

@@ -287,6 +287,9 @@ setup_kernels_arches()
                 fi
             fi
         fi
+        if [[ ! $arch ]]; then
+            die 12 $"Could not determine architecture."
+        fi
     fi
 
     # If only one arch is specified, make it so for all the kernels