Color PS1s and improve the rows and columns detection in serial shells

This overwrites the default debian bashrc with a custom bashrc that does
two things:

- Makes the prompt green or red depending on whether the previous
  command succeeded or failed, I think it's quite useful in general
- Before every command on the serial tty, uses a crazy escape sequence
  to query the host terminal for its dimensions and resize the guest
  terminal accordingly:
  https://blog.n-z.jp/blog/2022-01-30-serial-console-resize.html this
  makes ncurses apps like vim work fairly nicely even on the serial pane.
  Ideally, this should automatically run on host terminal resizes
  (SIGWINCH) but the series proposing this paravirtualization to QEMU
  never got reviewed and merged :(
  https://lists.endsoftwarepatents.org/archive/html/qemu-devel/2020-06/msg09591.html
  this should anyway be much better than the current situation where the
  guest always assumes an 80x24 screen

tasks.sh

@@ -252,6 +252,30 @@ mkdir /run/sshd/
 setsid /sbin/getty -l /bin/bash -n 115200 ${SERIAL_TTY}
 EOF
       chmod +x ${img_bind_mnt}/sbin/init-minimal
+      cat << EOF > ${img_bind_mnt}/etc/bash.bashrc
+# Use a green or red prompt depending on the previous command's exit value
+__prompt () {
+   if [[ $? = "0" ]]; then
+     PS1='\[\033[01;32m\]'
+   else
+     PS1='\[\033[01;31m\]'
+  fi
+  PS1="$PS1\u@\h:\w\$ \[\033[00m\]"
+}
+PROMPT_COMMAND=__prompt
+
+# On the serial tty, ask the host terminal for dimensions before each command
+__resize () {
+  local escape r c
+  IFS='[;' read -t 1 -sd R -p "$(printf '\e7\e[r\e[999;999H\e[6n\e8')" escape r c
+  if [[ "$r" -gt 0 && "$c" -gt 0 ]]; then
+    stty cols $c rows $r
+  fi
+}
+if [[ $TERM = "vt102" ]]; then
+  trap __resize DEBUG
+fi
+EOF
 
       # Atomically make the rootfs file available to unblock wait-for-vm tasks
       sync