include: kernel: Increase default stack size of all threads

Change to increase default stack size of all threads, to accomodate
dtb overlay implementation.

Change-Id: Ic1aedb99c19a33dd13d9ccbfc59913147018e252
diff --git a/include/kernel/thread.h b/include/kernel/thread.h
index 6c25712..5f8b42b 100644
--- a/include/kernel/thread.h
+++ b/include/kernel/thread.h
@@ -92,8 +92,8 @@
 #define DEFAULT_PRIORITY (NUM_PRIORITIES / 2)
 #define HIGH_PRIORITY ((NUM_PRIORITIES / 4) * 3)
 
-/* stack size */
-#define DEFAULT_STACK_SIZE 8192
+/* stack size  - 12kb per thread (12 * 1024) */
+#define DEFAULT_STACK_SIZE 12288
 
 /* functions */
 void thread_init_early(void);