Adds sdkconfig.local (intentionally not tracked in .git) for local overrides.

Here's what I have in mine, for example:

```
CONFIG_LOG_DEFAULT_LEVEL_WARN=n
CONFIG_LOG_DEFAULT_LEVEL_INFO=y
CONFIG_LOG_DEFAULT_LEVEL=3
```

.gitignore

@@ -20,3 +20,5 @@ test/dependencies.lock
 test/sdkconfig
 sdkconfig.bak
 *.ignore
+
+/sdkconfig.local

BUILDING.md

@@ -72,3 +72,15 @@ If you get errors involving missing C++ includes, then you may need to edit
 your editor's LSP invocation to include `--query-driver=**`.
 
 You should then get proper LSP integration via clangd.
+
+# ESP-IDF configuration
+
+Espressif exposes a large collection of configuration options[1] for its
+framework; you can use `idf.py menuconfig` to generate a custom `sdkconfig`
+file, eg. to change the logging level.
+
+To avoid needing to select the same set of options every time you regenerate
+the sdkconfig, you can also set some defaults in `sdkconfig.local`; this is not
+tracked in git, and is ideal for local / per-checkout changes.
+
+1. https://docs.espressif.com/projects/esp-idf/en/release-v3.3/api-reference/kconfig.html

CMakeLists.txt

@@ -5,7 +5,12 @@ cmake_minimum_required(VERSION 3.16)
 
 include($ENV{PROJ_PATH}/tools/cmake/common.cmake)
 
-set(SDKCONFIG_DEFAULTS "sdkconfig.common")
+get_filename_component(_abs_curr_dir "." ABSOLUTE)
+if (EXISTS ${_abs_curr_dir}/sdkconfig.local)
+  set(SDKCONFIG_DEFAULTS "sdkconfig.common;sdkconfig.local")
+else()
+  set(SDKCONFIG_DEFAULTS "sdkconfig.common")
+endif()
 
 # No exceptions in app builds (this is different in test builds).
 idf_build_set_property(COMPILE_OPTIONS "-DRESULT_DISABLE_EXCEPTIONS" APPEND)