feat: honour the boards a folder offers a core on

ROCKNIX writes some add_emu_core calls under a case on DEVICE, and the
rom folders file keeps that branch as the offer's `when` (uae4arm for
amiga, emulators/package.mk:188). A board's core list names such a core
when another folder offers it there, so --target placed its files in a
folder the board does not offer it for.

offer_boards() reads `when` and board_defaults against the platform's
targets, aliases included. A board build (generate_pack and generate
--manifest with --target, verify --target) carries the board in the
config (TARGET_KEY), and content_folder_tags leaves out the pairs the
board is not offered. A manifest for every board lists in
offered_boards each profile placed in such a folder, and install.py
--target drops its files there on the other boards.

Manifests: no file moves. The cores ROCKNIX offers under a device case
read no file beside the game today, so no manifest carries
offered_boards yet.

install/launchers/index.json

@@ -27,8 +27,8 @@
         {
           "dest": "roms/ports/retrobios/install.py",
           "repo_path": "install.py",
-          "size": 1220974,
-          "sha256": "67036bbe4db401de213d9a8ab44161b8b64bfde91834b438fff47802b5a461aa",
+          "size": 1222786,
+          "sha256": "8c8ead297c502f40f02f4407bdbb3edcb2af31b68f0a3fca06ed963c6cbc3cb0",
           "executable": false
         },
         {
@@ -71,8 +71,8 @@
     "muos": {
       "archive": {
         "dest": "ARCHIVE/RetroBIOS.muxapp",
-        "size": 1274599,
-        "sha256": "8ea3d79c9bbcec747555c3970edef0692db5a00ada0ff45892aaf68dcfc38eb6"
+        "size": 1276411,
+        "sha256": "8008566a12fc1d18a19d9510c1884919d60ede9eb227bef25973082ac1b204e3"
       },
       "members": [
         {
@@ -92,8 +92,8 @@
         {
           "name": "RetroBIOS/install.py",
           "repo_path": "install.py",
-          "size": 1220974,
-          "sha256": "67036bbe4db401de213d9a8ab44161b8b64bfde91834b438fff47802b5a461aa",
+          "size": 1222786,
+          "sha256": "8c8ead297c502f40f02f4407bdbb3edcb2af31b68f0a3fca06ed963c6cbc3cb0",
           "executable": false
         },
         {
@@ -166,8 +166,8 @@
         {
           "dest": ".config/modules/retrobios/install.py",
           "repo_path": "install.py",
-          "size": 1220974,
-          "sha256": "67036bbe4db401de213d9a8ab44161b8b64bfde91834b438fff47802b5a461aa",
+          "size": 1222786,
+          "sha256": "8c8ead297c502f40f02f4407bdbb3edcb2af31b68f0a3fca06ed963c6cbc3cb0",
           "executable": false
         },
         {