idf.py: guess IDF_TARGET from sdkconfig if CMakeCache doesn't exist

This solves the issue that target is changed to the default one after
idf.py fullclean.

Also allow setting the default target using sdkconfig.defaults, e.g.
CONFIG_IDF_TARGET="esp32s2"

Closes IDF-1040

tools/idf_py_actions/tools.py

@@ -171,6 +171,18 @@ def ensure_build_directory(args, prog_name, always_run_cmake=False):
     if not os.path.isdir(build_dir):
         os.makedirs(build_dir)
     cache_path = os.path.join(build_dir, "CMakeCache.txt")
+    if not os.path.exists(cache_path) and not os.environ.get("IDF_TARGET"):
+        # CMakeCache.txt does not exist yet, and IDF_TARGET is not set in the environment.
+        # Try to guess the target from sdkconfig, so that it doesn't get reset to the default ("esp32").
+        sdkconfig_path = os.path.join(args.project_dir, "sdkconfig")
+        # Also consider the CONFIG_IDF_TARGET value in sdkconfig.defaults.
+        sdkconfig_defaults_path = os.path.join(args.project_dir, "sdkconfig.defaults")
+        idf_target_from_sdkconfig = (get_sdkconfig_value(sdkconfig_path, "CONFIG_IDF_TARGET") or
+                                     get_sdkconfig_value(sdkconfig_defaults_path, "CONFIG_IDF_TARGET"))
+        if idf_target_from_sdkconfig:
+            if args.verbose:
+                print("IDF_TARGET is not set, guessed '%s' from sdkconfig" % (idf_target_from_sdkconfig))
+            args.define_cache_entry.append("IDF_TARGET=" + idf_target_from_sdkconfig)
 
     args.define_cache_entry.append("CCACHE_ENABLE=%d" % args.ccache)
 
@@ -233,3 +245,23 @@ def merge_action_lists(*action_lists):
         merged_actions["actions"].update(action_list.get("actions", {}))
         merged_actions["global_action_callbacks"].extend(action_list.get("global_action_callbacks", []))
     return merged_actions
+
+
+def get_sdkconfig_value(sdkconfig_file, key):
+    """
+    Return the value of given key from sdkconfig_file.
+    If sdkconfig_file does not exist or the option is not present, returns None.
+    """
+    assert key.startswith("CONFIG_")
+    if not os.path.exists(sdkconfig_file):
+        return None
+    # keep track of the last seen value for the given key
+    value = None
+    # if the value is quoted, this excludes the quotes from the value
+    pattern = re.compile(r"^{}=\"?([^\"]*)\"?$".format(key))
+    with open(sdkconfig_file, "r") as f:
+        for line in f:
+            match = re.match(pattern, line)
+            if match:
+                value = match.group(1)
+    return value