Change default cargo target

Also, use gdb-multiarch as cargo runner, as it's the easiest for Debian.

.cargo/config

@@ -1,5 +1,8 @@
 [target.thumbv7em-none-eabihf]
-runner = 'arm-none-eabi-gdb'
+#runner = 'arm-none-eabi-gdb'
+runner = 'gdb-multiarch'
 rustflags = [
   "-C", "link-arg=-Tlink.x", "-C", "inline-threshold=255",
 ]
+[build]
+target = "thumbv7em-none-eabihf"