pvmfw: Prepare heap.rs for move

Allow client code to control the size of the HEAP through a macro.

Make the SAFETY comment a compliant docstring.

Test: TH
Change-Id: I386e0c2e070e8c96f90560881005af827607fecc
diff --git a/pvmfw/src/entry.rs b/pvmfw/src/entry.rs
index 0d2dfda..2582d55 100644
--- a/pvmfw/src/entry.rs
+++ b/pvmfw/src/entry.rs
@@ -15,6 +15,7 @@
 //! Low-level entry and exit points of pvmfw.
 
 use crate::config;
+use crate::configure_global_allocator_size;
 use crate::crypto;
 use crate::fdt;
 use crate::heap;
@@ -36,7 +37,7 @@
     console,
     layout::{self, crosvm},
     logger, main,
-    memory::{min_dcache_line_size, MemoryTracker, MEMORY, SIZE_4KB},
+    memory::{min_dcache_line_size, MemoryTracker, MEMORY, SIZE_128KB, SIZE_4KB},
     power::reboot,
 };
 use zeroize::Zeroize;
@@ -62,6 +63,7 @@
 }
 
 main!(start);
+configure_global_allocator_size!(SIZE_128KB);
 
 /// Entry point for pVM firmware.
 pub fn start(fdt_address: u64, payload_start: u64, payload_size: u64, _arg3: u64) {