layout: Improve development workflow

Right now, if layout.ld or target.json is changed, the firmware will
not automatically relink when running "cargo xbuild". This can make
working on layout.ld quite tedious, as you have to manually change the
Rust code to get the firmware to rebuild.

We solve this by adding a tiny build script which simply rebuilds the
program if either layout.ld or target.json changes.

We also stop using "-s" (aka "--strip-all") in the target options.
Instead, we add a second /DISCARD/ section in the linker script. This
puts all of our logic about sections in a single place. It also makes
it easier for a developer to get the symbols (if they want them).

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

build.rs

@@ -0,0 +1,4 @@
+fn main() {
+    println!("cargo:rerun-if-changed=target.json");
+    println!("cargo:rerun-if-changed=layout.ld");
+}

layout.ld

@@ -51,4 +51,9 @@ SECTIONS
 		*(.comment)
 		*(COMMON)
 	}
+	/* Strip symbols from the output binary (comment out to get symbols) */
+	/DISCARD/ : {
+		*(.symtab)
+		*(.strtab)
+	}
 }

target.json

@@ -16,9 +16,6 @@
   "code-model": "small",
   "relocation-model": "static",
   "pre-link-args": {
-    "ld.lld": [
-      "-s",
-      "--script=layout.ld"
-    ]
+    "ld.lld": ["--script=layout.ld"]
   }
 }