esp32/boards/ESP32_GENERIC_S3: Reinstate old FLASH_4M variant.

Commit 6201e77 removed the
ESP32_GENERIC_S3 FLASH_4M variant from `board.json`.  But the firmware
still exists on the download server, and it makes sense to still keep those
old versions available for download, just like all other older versions are
still available.

This commit introduce a new scheme for `board.json` whereby old variants
that are no longer built can be moved to the "old_variants" section.  This
keeps them available on the download page, allowing a way to deprecate
individual board variants without removing them entirely.

An optional string can be added to the old variant to describe why it's
obsolete and what to use instead.

Signed-off-by: Damien George <[email protected]>

Author: dpgeorge

ports/esp32/boards/ESP32_GENERIC_S3/board.json

@@ -22,5 +22,8 @@
     "vendor": "Espressif",
     "variants": {
         "SPIRAM_OCT": "Support for Octal-SPIRAM"
+    },
+    "old_variants": {
+        "FLASH_4M": ["4MiB flash", "Use the standard variant instead."]
     }
 }