configs: Use .SECONDARY to prevent deletion of intermediate files

Cleaner than the dummy rule.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
inmates: Generalize inmate makefile rules

This reduces the required changes for adding a new inmate to extending
the INMATES list and creating a my-inmate-y object list.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>