ci_set_matrix: split a port into alphabetical runs, not interleaved

Each part covered the whole alphabet, so finding a board meant looking
at every part. Slice the sorted list into consecutive runs instead.

Author: lynt-smitka

tools/ci_set_matrix.py

@@ -271,17 +271,18 @@ def get_settings(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 a bigger port into alphabetical runs of equal size, 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]
+            size = math.ceil(len(boards) / parts)
+            for index, start in enumerate(range(0, len(boards), size), start=1):
+                name = f"{port}-{index}"
+                port_to_boards_to_build[name] = boards[start : start + size]
                 split_ports[name] = port
 
     if port_to_boards_to_build: