Detect firmware/sdkconfig.defaults.local at configure time and feed it
into ESP-IDF's SDKCONFIG_DEFAULTS list alongside sdkconfig.defaults.
This gives self-hosted and downstream builds a first-class way to pin
per-deployment values (CONFIG_STACKCHAN_SERVER_URL, CONFIG_OTA_URL,
CONFIG_USE_EZDATA, ...) without patching committed defaults and
without needing contributors to remember the
SDKCONFIG_DEFAULTS="sdkconfig.defaults;sdkconfig.defaults.local"
invocation.
The file is ignored by the repo's .gitignore, so it never leaks
upstream. When it does not exist, behaviour is unchanged.
A status message ("StackChan: detected sdkconfig.defaults.local,
applying overlay") makes the override visible in the idf.py configure
output so it's obvious when a build is using custom defaults.
Forairaaaaa
Nourrisse Florian