ci: split a port with more than 256 boards into several matrix jobs

GitHub allows 256 configurations per matrix and build-boards.yml runs
one matrix per port. espressif reached 257 boards, so every full build
fails with "Strategy for job 'board' produced 257 configurations".

ci_set_matrix.py now splits such a port into equal parts listed like
ports (espressif-1, espressif-2) and a "split_ports" map lets build.yml
hand the real port name to build-boards.yml, so the toolchain setup is
unchanged. Ports below the limit and the per-board job names are not
affected.

Fixes adafruit#11324

Author: lynt-smitka

.github/workflows/build.yml

@@ -231,4 +231,4 @@ jobs:
     with:
       boards: ${{ toJSON(fromJSON(needs.scheduler.outputs.ports)[matrix.port]) }}
       cp-version: ${{ needs.scheduler.outputs.cp-version }}
-      port: ${{ matrix.port }}
+      port: ${{ fromJSON(needs.scheduler.outputs.ports).split_ports[matrix.port] || matrix.port }}

tools/ci_set_matrix.py

@@ -23,6 +23,7 @@
 
 import re
 import os
+import math
 import sys
 import json
 import pathlib
@@ -68,6 +69,8 @@
     r"(?:-stubs|\.(?:md|MD|mk|rst|RST)|/Makefile)$"
 )
 
+GITHUB_MATRIX_LIMIT = 256
+
 PATTERN_WINDOWS = {
     ".github/",
     "extmod/",
@@ -267,8 +270,23 @@ def get_settings(board):
         port_to_boards_to_build.setdefault(port, []).append(board)
         print(" ", board)
 
+    # build-boards.yml runs one matrix per port and GitHub allows 256 jobs per matrix.
+    # Split a bigger port into equal parts, listed like ports; "split_ports" maps a part
+    # back to the real port name, which build.yml passes on, so the toolchain setup in
+    # build-boards.yml stays unchanged.
+    split_ports = {}
+    for port, boards in list(port_to_boards_to_build.items()):
+        parts = math.ceil(len(boards) / GITHUB_MATRIX_LIMIT)
+        if parts > 1:
+            del port_to_boards_to_build[port]
+            for index in range(parts):
+                name = f"{port}-{index + 1}"
+                port_to_boards_to_build[name] = boards[index::parts]
+                split_ports[name] = port
+
     if port_to_boards_to_build:
         port_to_boards_to_build["ports"] = sorted(list(port_to_boards_to_build.keys()))
+        port_to_boards_to_build["split_ports"] = split_ports
 
     # Set the step outputs
     set_output("ports", json.dumps(port_to_boards_to_build))