cortexm: bump default stack size to 2048 bytes

Previously it was 1024 bytes, which occasionally ran into a stack
overflow. I hope that 2048 bytes will be enough for most purposes.

I've also removed some 2048-byte stack size settings in JSON files,
which are unnecessary now that the parent (cortex-m.json) sets them.

targets/pyportal.json

@@ -4,6 +4,5 @@
     "flash-1200-bps-reset": "true",
     "flash-method": "msd",
     "msd-volume-name": "PORTALBOOT",
-    "msd-firmware-name": "firmware.uf2",
-    "default-stack-size": 2048
+    "msd-firmware-name": "firmware.uf2"
 }