scripts/spdxcheck: Add option to display files without SPDX

Makes life easier when chasing the missing ones. Is activated with '-f'
on the command line.

# scripts/spdxcheck.py -f kernel/
Files without SPDX:
    ./kernel/cpu.c
    ./kernel/kmod.c
    ./kernel/relay.c
    ./kernel/bpf/offload.c
    ./kernel/bpf/preload/.gitignore
    ./kernel/bpf/preload/iterators/README
    ./kernel/bpf/ringbuf.c
    ./kernel/cgroup/cgroup.c
    ./kernel/cgroup/cpuset.c
    ./kernel/cgroup/legacy_freezer.c
    ./kernel/debug/debug_core.h
    ./kernel/debug/kdb/Makefile
    ./kernel/debug/kdb/kdb_bp.c
    ./kernel/debug/kdb/kdb_bt.c
    ./kernel/debug/kdb/kdb_cmds
    ./kernel/debug/kdb/kdb_debugger.c
    ./kernel/debug/kdb/kdb_io.c
    ./kernel/debug/kdb/kdb_keyboard.c
    ./kernel/debug/kdb/kdb_main.c
    ./kernel/debug/kdb/kdb_private.h
    ./kernel/debug/kdb/kdb_support.c
    ./kernel/locking/lockdep_states.h
    ./kernel/locking/mutex-debug.c
    ./kernel/locking/spinlock_debug.c
    ./kernel/sched/pelt.h

With the optional -D parameter the directory depth can be limited:

# scripts/spdxcheck.py -f -D 0 kernel/
Files without SPDX:
    ./kernel/cpu.c
    ./kernel/kmod.c
    ./kernel/relay.c

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>