Fix address-truthiness bugs around a valid 0x0

[ppannuto-claude]

Summary

Several places in tockloader distinguish "no address given/known" from
"an address was given/known" using plain truthiness (if addr:) on a
value that is None when unset. That conflates "unset" with an address
of exactly 0x0, which is a normal, valid flash and/or app base address
(e.g. any Cortex-M board, or Tock's qemu_arm_mps2 boards) — found
while getting qemu_arm_mps2 working with tockloader local-board.

Fixed all instances of this pattern to check is not None instead:

• main.py command_local_board_set(): an explicit --app-address 0x0/--flash-address 0x0 was validated but then silently omitted
  from the status output, making it look like the flag was ignored.
• flash_file.py set_local_board(): the same values were then
  silently dropped from the persisted local-board TOML config,
  discarding the deliberate setting entirely.
• flash_file.py FlashFile.get_flash_address(): a stored
  flash_address == 0 was reported as None (unknown) instead of 0.
• board_interface.py BoardInterface.get_apps_start_address(): a
  configured app_address == 0 was ignored in favor of querying the
  board's appaddr attribute (or returning None if that's absent).
• tockloader.py _get_apps_start_address(): a cached value of 0,
  an explicit --app-address 0x0 on any command (not just
  local-board set), or a channel-reported address of 0 were all
  treated as "unknown" and silently overridden by lower-priority
  defaults.
• tockloader.py _get_flash_start_address(): a channel-reported
  flash address of 0 was treated as "unknown" and replaced with the
  function's own hardcoded 0 default -- harmless today only because
  that default happens to also be 0.
• tockloader.py _get_memory_start_address(): same cached-value
  pattern as above, for the app RAM start address.

Note: a related but distinct bug in _reshuffle_apps() (a debug-log
format string crashing on a None RAM address) is being tracked
separately upstream as tock#141 and is not part of this PR.

Test plan

• python3 -m tockloader.main local-board set zerotest --arch cortex-m3 --app-address 0x0 --flash-address 0x0 — confirmed both
  values now appear in the status output and are correctly
  persisted as 0 (not omitted) in the TOML config.
• No project test suite exists to run (CI only builds + smoke-tests
  --version/list-known-boards). The remaining sites
  (_get_apps_start_address, _get_flash_start_address,
  _get_memory_start_address, get_apps_start_address) were
  reviewed by inspection and are the same fix pattern, but not
  individually exercised against real hardware/QEMU in this
  session.

AI use disclosure

This PR was written primarily by Claude Code (Anthropic's Claude Sonnet
5). Claude was given a human-authored bug report describing the
original local-board truthiness bug with exact file/line locations,
then asked to audit the rest of the codebase for the same bug class,
which surfaced the additional sites above; a human then directed which
fixes to apply and how to structure the commit. Reviewed by the
submitter before opening/updating.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

🤖 Generated with Claude Code

https://claude.ai/code/session_01KTQ6pWrckM3jp9vfBo4bhk