flash.py: Auto-download from GitHub with cache, add --release/--offline flags

Default behavior now checks GitHub for the latest release and downloads
firmware if the cache is empty or outdated. Cached binaries are stored
in .firmware_cache/{board}/ with SHA-256 integrity verification.

New flags:
  --release TAG  Flash a specific release version
  --offline      Skip online check, use cached/local firmware only

Removed --download flag (downloading is now the default).
Added .firmware_cache/ and rnodethv3_firmware.bin to .gitignore.

.gitignore

@@ -3,6 +3,8 @@
 *.pyc
 TODO
 rnodethv4_firmware.bin
+rnodethv3_firmware.bin
+.firmware_cache/
 Release/*.hex
 Release/*.zip
 Release/*.json