Add debug log support for serial interface.

This shows the chatter on the serial interface if debug
logging is enabled.

```
RUST_LOG=debug target/debug/hostcontrol
```

Signed-off-by: Henner Zeller <h.zeller@acm.org>

hostcontrol/Cargo.lock

@@ -1446,6 +1446,7 @@ dependencies = [
  "eframe",
  "egui",
  "env_logger",
+ "log",
  "nalgebra",
  "nalgebra-macros",
  "serialport",
@@ -1759,9 +1760,9 @@ dependencies = [
 
 [[package]]
 name = "log"
-version = "0.4.27"
+version = "0.4.29"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "13dc2df351e3202783a1fe0d44375f7295ffb4049267b0f3018346dc122a1d94"
+checksum = "5e5032e24019045c762d3c0f28f5b6b8bbf38563a65908389bf7978758920897"
 
 [[package]]
 name = "mach2"