kernel: rename Z_KERNEL_STACK_BUFFER to K_KERNEL_STACK_BUFFER

Simple rename to align the kernel naming scheme. This is being
used throughout the tree, especially in the architecture code.
As this is not a private API internal to kernel, prefix it
appropriately with K_.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
diff --git a/soc/espressif/esp32/esp32-mp.c b/soc/espressif/esp32/esp32-mp.c
index c5ec588..baaae70 100644
--- a/soc/espressif/esp32/esp32-mp.c
+++ b/soc/espressif/esp32/esp32-mp.c
@@ -258,12 +258,12 @@
 
 	sr.cpu = cpu_num;
 	sr.fn = fn;
-	sr.stack_top = Z_KERNEL_STACK_BUFFER(stack) + sz;
+	sr.stack_top = K_KERNEL_STACK_BUFFER(stack) + sz;
 	sr.arg = arg;
 	sr.vecbase = vb;
 	sr.alive = &alive_flag;
 
-	appcpu_top = Z_KERNEL_STACK_BUFFER(stack) + sz;
+	appcpu_top = K_KERNEL_STACK_BUFFER(stack) + sz;
 
 	start_rec = &sr;