nexus sim: Adjust shodan toplevel load addresses

Nexus defines a different CPIO load address than shodan, so we need to
be able to change it during simulation time.

Change-Id: Ie8dcc08b336e8dcc7a9929a8c3fa5f2f7418bb15

shodan.resc

diff --git a/shodan.resc b/shodan.resc
index 998f3fa..cc1eff6 100644
--- a/shodan.resc
+++ b/shodan.resc
@@ -46,10 +46,12 @@
 
 # Load the tarball of CAmkES components, cpio bundle of built-in files, and the seL4 kernel
 $tar ?= @out/ext_flash_release.tar
+$tar_load_address ?= 0x44000000
 $cpio ?= @out/cantrip/riscv32-unknown-elf/release/ext_builtins.cpio
+$cpio_load_address ?= 0x46000000
 $kernel ?= @out/cantrip/riscv32-unknown-elf/release/kernel/kernel.elf
-sysbus LoadBinary $tar 0x44000000
-sysbus LoadBinary $cpio 0x46000000
+sysbus LoadBinary $tar $tar_load_address
+sysbus LoadBinary $cpio $cpio_load_address
 sysbus LoadSymbolsFrom $kernel
 
 # Start cpu0 at the bootrom reset vector, which is stored immediately after the