Before this patch you get tools/power/acpi/Makefile.rules
included in parallel trying to copy KERNEL_INCLUDE multiple
times:

make -j20 acpi
  DESCEND  power/acpi
  DESCEND  tools/acpidbg
  DESCEND  tools/acpidump
  DESCEND  tools/ec
  MKDIR    include
  MKDIR    include
  MKDIR    include
  CP       include
  CP       include
cp: cannot create directory '/home/abuild/rpmbuild/BUILD/linux-5.7.7+git20200917.10b82d51/tools/power/acpi/include/acpi': File exists
make[2]: *** [../../Makefile.rules:20: /home/abuild/rpmbuild/BUILD/linux-5.7.7+git20200917.10b82d51/tools/power/acpi/include] Error 1
make[1]: *** [Makefile:16: acpidbg] Error 2
make[1]: *** Waiting for unfinished jobs....

with this patch each subdirectory will be processed serialized:

  DESCEND  power/acpi
  DESCEND  tools/acpidbg
  MKDIR    include
  CP       include
  CC       tools/acpidbg/acpidbg.o
  LD       acpidbg
  STRIP    acpidbg
  DESCEND  tools/acpidump
  CC       tools/acpidump/apdump.o
...
  LD       acpidump
  STRIP    acpidump
  DESCEND  tools/ec
  CC       tools/ec/ec_access.o
  LD       ec
  STRIP    ec
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>