build/nexus: use hello_world for a model in order to fit in memory

On real hw we load the builtins bundle into a .5M carve-out of DRAM.
IREE models are too big to fit so include hello_world so there's something
for mltest to run.

Change-Id: I546a56926e50e01f5e883f268e4effe04f273b0f

platforms/nexus/cantrip_builtins.mk

diff --git a/platforms/nexus/cantrip_builtins.mk b/platforms/nexus/cantrip_builtins.mk
index 7990938..7ecbebf 100644
--- a/platforms/nexus/cantrip_builtins.mk
+++ b/platforms/nexus/cantrip_builtins.mk
@@ -20,7 +20,8 @@
                          $(CANTRIP_OUT_RUST_APP_RELEASE)/panic/panic.app \
                          $(CANTRIP_OUT_C_APP_RELEASE)/suicide/suicide.app \
                          $(CANTRIP_OUT_RUST_APP_RELEASE)/timer/timer.app
-CANTRIP_MODEL_RELEASE := $(OUT)/kelvin_iree/sparrow_iree/samples/microbenchmarks/conv1x1_test_emitc_static
+# Temporarily have only 5M for builtins; not enough for an IREE model
+CANTRIP_MODEL_RELEASE := $(OUT)//kelvin/sw/bazel_out/hello_world.kelvin
 
 CANTRIP_APPS_DEBUG    := $(CANTRIP_OUT_C_APP_DEBUG)/hello/hello.app \
                          $(CANTRIP_OUT_RUST_APP_DEBUG)/fibonacci/fibonacci.app \
@@ -30,9 +31,10 @@
                          $(CANTRIP_OUT_RUST_APP_DEBUG)/panic/panic.app \
                          $(CANTRIP_OUT_C_APP_DEBUG)/suicide/suicide.app \
                          $(CANTRIP_OUT_RUST_APP_DEBUG)/timer/timer.app
+# NB: debug builds only run on Renode where we have 16M for builtins
 CANTRIP_MODEL_DEBUG   := $(OUT)/kelvin_iree/sparrow_iree/samples/microbenchmarks/conv1x1_test_emitc_static
 
 CANTRIP_SCRIPTS       := $(ROOTDIR)/build/platforms/$(PLATFORM)/builtins.repl
 
-$(CANTRIP_MODEL_RELEASE): iree_model_builtins
+$(patsubst %.kelvin,%.elf,$(CANTRIP_MODEL_RELEASE)): kelvin_sw
 $(CANTRIP_MODEL_DEBUG): iree_model_builtins