fix(ci): restore board target matrix generation after zenoh Kconfig change

Since b11e615810 src/modules/zenoh/Kconfig sources the generated topic
catalog through ZENOH_KCONFIG_TOPICS, which only cmake exports, so every
standalone Kconfig parse crashed and build_all_targets produced an empty
build matrix on every run while the scan job still reported success.

Add Tools/kconfig/loadconfig.py as the single entry point for parsing
the Kconfig tree outside cmake: it generates the topic catalog exactly
like cmake/kconfig.cmake and provides shared board/target enumeration
and config loading. Port generate_board_targets_json.py (CI matrix) and
updateconfig.py (make updateconfig) to it; both were independently
broken and each reimplemented board scanning on its own.

Harden the workflow so the next failure of this class is loud: run the
generator as a standalone assignment, so bash -e fails the step (the
previous echo "matrix=$(...)" form masked the exit code), and write
the JSON outputs through the heredoc GITHUB_OUTPUT form.

Matrix output verified byte-identical to the pre-b11e6158101 baseline
in all three modes; updateconfig output verified identical against the
same baseline.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/build_all_targets.yml

@@ -86,17 +86,34 @@ jobs:
       - name: Install Python Dependencies
         run: pip3 install -U packaging -r ./Tools/setup/requirements.txt
 
+      # The generator runs as a standalone assignment so a non-zero exit
+      # fails the step; embedded in `echo "x=$(...)"` it would be masked
+      # and the empty output silently skips the whole build matrix.
       - id: set-matrix
         name: Generate Build Matrix
-        run: echo "matrix=$(./Tools/ci/generate_board_targets_json.py --group)" >> $GITHUB_OUTPUT
+        run: |
+          matrix=$(./Tools/ci/generate_board_targets_json.py --group)
+          {
+            echo "matrix<<EOF"
+            echo "$matrix"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
 
       - id: set-seeders
         name: Generate Seeder Matrix
-        run: echo "seeders=$(./Tools/ci/generate_board_targets_json.py --group --seeders)" >> $GITHUB_OUTPUT
+        run: |
+          seeders=$(./Tools/ci/generate_board_targets_json.py --group --seeders)
+          {
+            echo "seeders<<EOF"
+            echo "$seeders"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
 
       - id: set-timestamp
         name: Save Current Timestamp
-        run: echo "timestamp=$(date +"%Y%m%d%H%M%S")" >> $GITHUB_OUTPUT
+        run: |
+          timestamp=$(date +"%Y%m%d%H%M%S")
+          echo "timestamp=$timestamp" >> "$GITHUB_OUTPUT"
 
       - id: set-branch
         name: Save Current Branch Name