freertos: Bump default idle task stack size to 1536 bytes

freertos: Bump default idle task stack size to 1536 bytes

In situations where idle task runs a lot of idle hooks or cleanup code
(due to pthread local storage, etc) it can use more than 1KB of stack.

(I think the trigger is if a context switch happens at the right point
in the TLS cleanup).

Also removes an sdkconfig.default which accidentally set all config items,
including this stack size.