build: make depclean remove the userspace dependency files too

[greatEndian]

Problem

src/Makefile's depclean target removes only depends, which holds the
realtime dependency files. The userspace ones are written next to their
objects as objects/**/*.d — see TODEPS (Makefile:229) and the
-MF "${@:.o=.d}" compile rules (Makefile:291, 301, 309, 317) — and survive
it. A typical run-in-place tree keeps 535 of them.

The target's own comment claims otherwise ("clean cleans everything but
dependency files, and depclean cleans them too"), and that comment is wrong
in both directions: clean does remove the userspace dependency files,
because genclean deletes objects/ wholesale.

Why it matters

A stale dependency file still declares its original source:

objects/hal/utils/halrmt.o: hal/utils/halrmt.c

gcc's -MP writes dummy targets for the headers only, never for the main
source, so once hal/utils/halrmt.c has been renamed the prerequisite cannot
be satisfied and the entire build stops:

make: *** No rule to make target 'hal/utils/halrmt.c',
needed by 'objects/hal/utils/halrmt.o'.  Stop.

The documented remedy for stale dependency information is make depclean —
and it does not work. The only way out is make clean and a full rebuild.

This is not hypothetical. It was hit on a run-in-place tree carried across two
upstream changes: the halrmt.c -> halrmt.cc rename, and the
src/libnml/posemath -> src/libposemath move. Eight dependency files pointed
at sources that no longer existed, and the tree could not be built at all.

Fix

Have depclean remove objects/**/*.d as well, and correct the comment.

Removing them costs no recompilation: nothing takes a .d as a prerequisite,
and UNREAD_DEPS (Makefile:255) is computed but never used. The only thing
given up is header-dependency tracking until each object is next rebuilt,
which is precisely what asking for depclean means.

Verification

• Planted a dependency file naming a since-renamed source and reproduced the
  failure verbatim, including the -MP subtlety (a first attempt that included
  a dummy rule for the main source did not reproduce it, which is what
  confirmed the mechanism).
• Before the change: make depclean left all 535 objects/**/*.d in place and
  the build still aborted.
• After the change: make depclean removed all 535, and the previously fatal
  target built normally.
• No forced rebuild: a full make default immediately after a patched
  depclean on an up-to-date tree recompiled 0 files.