Warn if attempting to use ACK with TRUSTY

Author: Gulshan Singh

Makefile

@@ -464,6 +464,12 @@ trusty_clean:
 
 # When Trusty is enabled, use the kernel image built by Trusty
 ifeq ($(TRUSTY),1)
+  ifeq ($(ACK),1)
+    ifndef QEMU_KERNEL_IMAGE
+      $(warning $(YELLOW)ACK was enabled, but Trusty runs with its own kernel. To force using a specific kernel image, add `QEMU_KERNEL_IMAGE=$(KERNEL_IMAGE)` to the command line $(NC))
+    endif
+  endif
+
   KERNEL_IMAGE := $(TRUSTY_KERNEL_IMAGE)
 endif