Fix nexus platform.mk prereqs setting

Also update the order of the host tools to ensure they can be properly
referenced.

Change-Id: I3a4b4ff9ac5f69bc7a9222d7ac7343c14d54250d

platforms/nexus/platform.mk

diff --git a/platforms/nexus/platform.mk b/platforms/nexus/platform.mk
index 34ea006..8dd2c2e 100644
--- a/platforms/nexus/platform.mk
+++ b/platforms/nexus/platform.mk
@@ -1,3 +1,12 @@
+# Platform-specific requirements handled by "m prereqs"
+CANTRIP_PLATFORM_PYTHON_DEPS=\
+    ${ROOTDIR}/hw/opentitan-upstream/python-requirements.txt
+CANTRIP_PLATFORM_APT_DEPS=\
+    ${ROOTDIIR}/hw/opentitan-upstream/apt-requirements.txt
+
+# Put host tool targets first.
+include $(ROOTDIR)/build/platforms/nexus/renode.mk
+
 include $(ROOTDIR)/build/platforms/nexus/boot_rom.mk
 include $(ROOTDIR)/build/platforms/nexus/cantrip.mk
 include $(ROOTDIR)/build/platforms/nexus/cantrip_builtins.mk
@@ -5,7 +14,6 @@
 include $(ROOTDIR)/build/platforms/nexus/kelvin.mk
 include $(ROOTDIR)/build/platforms/nexus/opentitan_sw.mk
 include $(ROOTDIR)/build/platforms/nexus/opentitan_hw.mk
-include $(ROOTDIR)/build/platforms/nexus/renode.mk
 include $(ROOTDIR)/build/platforms/nexus/matcha_hw.mk
 include $(ROOTDIR)/build/platforms/nexus/tock.mk