defconfig: Enable more debug configs on SDXPOORWILLS target

- FREELIST_RANDOM to control the state of the heap and reduce risk
  surface
- DEBUG_CREDENTIALS to debugging of credential management.
- Enable config to trigger panic on data corruption.

Change-Id: I82365c2c7d347600859b20dd18c4b8dede9256e0
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>