change addresses used by nonvolatile component

Used sstorage and estorage for the kernel address and length,
as suggested. Changed the userspace address and length to
0x08038000 and 0x2000 in src/main.rs and chip_layout.ld to
avoid overwriting the code segment

boards/stm32f3discovery/chip_layout.ld

@@ -1,13 +1,14 @@
 /* Memory layout for the STM32F303VCT6
  * rom = 256KB (LENGTH = 0x00040000)
  * kernel = 128KB
- * user = 128KB
+ * user = 96KB
+ * user storage space = 32KB (defined in src/main.rs)
  * ram = 48KB */
 
 MEMORY
 {
   rom (rx)  : ORIGIN = 0x08000000, LENGTH = 0x00020000
-  prog (rx) : ORIGIN = 0x08020000, LENGTH = 0x00020000
+  prog (rx) : ORIGIN = 0x08020000, LENGTH = 0x00018000
   ram (rwx) : ORIGIN = 0x20000000, LENGTH = 48K
 }
 

boards/stm32f3discovery/src/main.rs

@@ -608,14 +608,22 @@ pub unsafe fn reset_handler() {
         )
     );
     stm32f303xc::adc::ADC1.set_client(adc);
+
+    // Kernel storage region, allocated with the storage_volume!
+    // macro in common/utils.rs
+    extern "C" {
+        /// Beginning on the ROM region containing app images.
+        static _sstorage: u8;
+        static _estorage: u8;
+    }
 
     let nonvolatile_storage = components::nonvolatile_storage::NonvolatileStorageComponent::new(
         board_kernel,
         &stm32f303xc::flash::FLASH,
-        0x08020000,
-        0x20000,
-        0x08000000,
-        0x20000,
+        0x08038000,
+        0x2000,
+        &_sstorage as *const u8 as usize,
+        &_estorage as *const u8 as usize - &_sstorage as *const u8 as usize,
     )
     .finalize(components::nv_storage_component_helper!(
         stm32f303xc::flash::Flash