Review real.clean global Makefile target which deletes coreboot 3rdparty dir

Nobody has claimed this yet.

On a custom distribution, the coreboot build fails due to missing vboot headers.

```
$ git log --oneline --no-decorate -1
15c4bf9 config/coreboot-qemu-tpm2.config: Disable CCACHE
$ make PATH="/dev/shm/heads/install/x86/bin:$PATH" BOARD=qemu-coreboot-fbwhiptail-tpm2-hotp
[…]
2023-08-16 14:34:37+02:00 CPIO-XZ build/x86/qemu-coreboot-fbwhiptail-tpm2-hotp/initrd.cpio.xz
4126cdf615bf8c085cffef0cefe46e0e0ccc25d68d06e76190384a4913e71ff2 build/x86/qemu-coreboot-fbwhiptail-tpm2-hotp/initrd.cpio.xz
5794304:build/x86/qemu-coreboot-fbwhiptail-tpm2-hotp/initrd.cpio.xz
2023-08-16 14:34:45+02:00 MAKE coreboot
tail /dev/shm/heads/build/x86/log/coreboot.log
-----
from /dev/shm/heads/build/x86/coreboot-4.19/util/cbfstool/cbfstool.c:11:
/dev/shm/heads/build/x86/coreboot-4.19/src/commonlib/bsd/include/commonlib/bsd/cbfs_serialized.h:7:10: fatal error: vb2_sha.h: No such file or directory
7 | #include
| ^~~~~~~~~~~
compilation terminated.
In file included from /dev/shm/heads/build/x86/coreboot-4.19/util/cbfstool/common.h:12,
from /dev/shm/heads/build/x86/coreboot-4.19/util/cbfstool/elfparsing.h:7,
from /dev/shm/heads/build/x86/coreboot-4.19/util/cbfstool/cbfs-mkstage.c:8:
/dev/shm/heads/build/x86/coreboot-4.19/src/commonlib/bsd/include/commonlib/bsd/cbfs_serialized.h:7:10: fatal error: vb2_sha.h: No such file or directory
7 | #include
| ^~~~~~~~~~~
compilation terminated.
make[1]: *** [util/cbfstool/Makefile.inc:170: qemu-coreboot-fbwhiptail-tpm2-hotp/util/cbfstool/xdr.o] Error 1
make[1]: *** [util/cbfstool/Makefile.inc:170: qemu-coreboot-fbwhiptail-tpm2-hotp/util/cbfstool/cbfstool.o] Error 1
make[1]: *** [util/cbfstool/Makefile.inc:170: qemu-coreboot-fbwhiptail-tpm2-hotp/util/cbfstool/cbfs-mkstage.o] Error 1
#
# No change to /dev/shm/heads/build/x86/coreboot-4.19/qemu-coreboot-fbwhiptail-tpm2-hotp/.config
#
qemu-coreboot-fbwhiptail-tpm2-hotp/auto.conf:1568:notice: override: reassigning to symbol LAPIC_ACCESS_MODE
make[1]: Leaving directory '/dev/shm/heads/build/x86/coreboot-4.19'
make: *** [Makefile:461: /dev/shm/heads/build/x86/coreboot-4.19/qemu-coreboot-fbwhiptail-tpm2-hotp/.build] Error 1
```

Full log: [`build/x86/log/coreboot.log`](https://github.com/osresearch/heads/files/12359598/coreboot.log)

$ find . -iname vb2_sha.h
$

How should the submodules be retrieved?

Inspect the global Makefile's real.clean target and the coreboot retrieval or submodule handling it invokes. Reproduce the qemu-coreboot-fbwhiptail-tpm2-hotp build and review build/x86/log/coreboot.log, focusing on the missing vb2_sha.h error. Done means the target's behavior and required submodule retrieval are clarified and the reported build failure is addressed.

Written by the indexing model from the issue text.