Add JLink RTT feature

* SEGGER J-Link RTT allows to do terminal I/O without any additional
  connection except the J-Link itself.

See openocd patch (yet unmerged) at http://openocd.zylin.com/#/c/4055/8

Cargo.toml

@@ -22,6 +22,7 @@ default-target = "targets/aarch64-vesper-metta.json"
 unstable = []
 realtime = []
 noserial = []
+jlink = ['jlink_rtt']
 
 #[lib]
 #name = "nucleus"
@@ -36,9 +37,10 @@ bitflags = "1.0.1"
 register = "0.3.2"
 cortex-a = "2.4"
 #embedded-serial = "0.5.0"
+jlink_rtt = { version = "0.1.0", optional = true }
 
 [profile.dev]
-panic = "abort"
+panic = "abort" # @todo try panic_rtt when feature jlink
 
 [profile.release]
 panic = "abort"

Makefile

@@ -52,7 +52,7 @@ OPENOCD = /usr/local/openocd-20ceec69/bin/openocd
 all: kernel8.img
 
 target/$(TARGET)/release/vesper: $(SOURCES)
-	cargo xbuild --target=$(TARGET_JSON) --release
+	cargo xbuild --target=$(TARGET_JSON) --release --features="jlink"
 
 kernel8.img: target/$(TARGET)/release/vesper $(SOURCES)
 	cp $< ./kernel8

src/main.rs

@@ -33,7 +33,9 @@ pub mod platform;
 mod sync;
 mod write_to;
 
-// use core::fmt::Write;
+use core::fmt::Write;
+#[cfg(feature = "jlink")]
+use jlink_rtt::Output;
 use platform::{
     display::{Color, Size2d},
     gpio::GPIO,
@@ -78,6 +80,9 @@ fn kmain() -> ! {
         Err(_) => endless_sleep(),
     }
 
+    let mut out = Output::new();
+    writeln!(out, "JLink RTT is working!"); // @todo RttConsole
+
     println!("\n[0] UART is live!");
 
     extern "C" {