docs: Makefile: Inherit PYTHONPYCACHEPREFIX setting as env variable

docs: Makefile: Inherit PYTHONPYCACHEPREFIX setting as env variable

Commit 6c2f0b28d76e ("docs: Makefile: store __pycache__ at the output
directory") assigns a new path to PYTHONPYCACHEPREFIX for building
kernel documentation.

However, it is not necessarily optimal for everyone.

If you find PYTHONPYCACHEPREFIX is already set, it strongly suggests
that the developer has selected the setting as best suited for one's own
workflow.

Use "?=" in the assignment to PYTHONPYCACHEPREFIX so that the path of
$(abspath $(BUILDDIR)/__pycache__) works only as a safeguard.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <0253ce98-960c-4498-8ace-a4354e3ebc26@gmail.com>