start_item_riscv32: Correctly pass the app_heap_break to Rust code

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

core/src/entry_point/start_item_riscv32.rs

diff --git a/core/src/entry_point/start_item_riscv32.rs b/core/src/entry_point/start_item_riscv32.rs
index 9b57c76..347400c 100644
--- a/core/src/entry_point/start_item_riscv32.rs
+++ b/core/src/entry_point/start_item_riscv32.rs
@@ -88,7 +88,7 @@
     mv   a0, s0             // first arg is app_start
     mv   s0, sp             // Set the frame pointer to sp.
     mv   a1, s1             // second arg is stacktop
-    mv   a2, s2             // third arg is app_heap_break
+    mv   a2, t1             // third arg is app_heap_break that we told the kernel
     jal  rust_start"
     :                                                              // No output operands
     :