[S] Change in coreboot[master]: doc/Makefile: Fix build dir setting

[Nico Huber (Code Review)]

Attention is currently required from: Elyes Haouas, Felix Held, Felix Singer, Jan Samek, Martin L Roth.

Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/75893?usp=email )

Change subject: doc/Makefile: Fix build dir setting
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Patch Set 4: -Code-Review

(2 comments)

Commit Message:

https://review.coreboot.org/c/coreboot/+/75893/comment/08c8b6de_bcace405 :
PS4, Line 16: and make it writable.
> I assume this makes no difference, at least commit 4d8da8ed alone […]

Forgot that I can just read how it's called: In `makeSphinx.sh` we have
```
make sphinx BUILDDIR=/tmp/build
```
This `BUILDDIR=/tmp/build` got through even to the `Makefile.sphinx`.
What broke it is the different variable name. Because rn. we have
an explicit `BUILDDIR=` in the `Makefile` overriding the one from
the `makeSphinx.sh`.

Patchset:

PS4: 
With the variable names the same, I don't understand the overall change
(before commit 4d8da8ed to after this one). The only effective difference
seems to be the added `mkdir -p`. Is this necessary?