kernel/spinlock: Predicate spinlock validation on flash size

The spinlock validation isn't super lightweight -- it adds only a few
tens of bytess per call, but there are a LOT of locking calls.  On
smaller platforms with 32kb of flash, we're bumping into code size
limits on the bigger tests (tests/kernel/poll is a particular
offender).

Check the declared flash size before enabling it.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

include/spinlock.h

@@ -24,6 +24,15 @@ static inline void _arch_irq_unlock(int key)
 }
 #endif
 
+/* There's a spinlock validation framework available when asserts are
+ * enabled.  It adds a relatively hefty overhead (about 3k or so) to
+ * kernel code size, don't use on platforms known to be small. (Note
+ * we're using the kconfig value here.  This isn't defined for every
+ * board, but the default of zero works well as an "infinity"
+ * fallback.  There is a DT_FLASH_SIZE parameter too, but that seems
+ * even more poorly supported.
+ */
+#if (CONFIG_FLASH_SIZE == 0) || (CONFIG_FLASH_SIZE > 32)
 #if defined(CONFIG_ASSERT) && (CONFIG_MP_NUM_CPUS < 4)
 #include <misc/__assert.h>
 struct k_spinlock;
@@ -31,6 +40,7 @@ int z_spin_lock_valid(struct k_spinlock *l);
 int z_spin_unlock_valid(struct k_spinlock *l);
 #define SPIN_VALIDATE
 #endif
+#endif
 
 struct k_spinlock_key {
 	int key;