sim: mcuboot-sys: 2018 edition fix

Automatic migration to Rust 2018:

    cargo fix --edition

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/mcuboot-sys/Cargo.toml b/sim/mcuboot-sys/Cargo.toml
index 701b1e0..9c39326 100644
--- a/sim/mcuboot-sys/Cargo.toml
+++ b/sim/mcuboot-sys/Cargo.toml
@@ -5,6 +5,7 @@
 description = "A simple wrapper around the mcuboot code."
 build = "build.rs"
 publish = false
+edition = "2018"
 
 [features]
 # By default, build with simplistic signature verification.
diff --git a/sim/mcuboot-sys/src/c.rs b/sim/mcuboot-sys/src/c.rs
index d828607..dc73134 100644
--- a/sim/mcuboot-sys/src/c.rs
+++ b/sim/mcuboot-sys/src/c.rs
@@ -1,9 +1,9 @@
 /// Interface wrappers to C API entering to the bootloader
 
-use area::AreaDesc;
+use crate::area::AreaDesc;
 use simflash::SimFlashMap;
 use libc;
-use api;
+use crate::api;
 use std::sync::Mutex;
 
 lazy_static! {
@@ -85,7 +85,7 @@
 }
 
 mod raw {
-    use area::CAreaDesc;
+    use crate::area::CAreaDesc;
     use libc;
 
     extern "C" {
diff --git a/sim/mcuboot-sys/src/lib.rs b/sim/mcuboot-sys/src/lib.rs
index 9f5e6b5..9f5c850 100644
--- a/sim/mcuboot-sys/src/lib.rs
+++ b/sim/mcuboot-sys/src/lib.rs
@@ -10,4 +10,4 @@
 // functions are exported to C code.
 pub mod api;
 
-pub use area::{AreaDesc, FlashId};
+pub use crate::area::{AreaDesc, FlashId};