Drop user to a clear warning/error message on <4GB systems

[probonopd]

Drop user to a clear warning/error message on <4GB systems in addition to:

"Enter full pathname of shell or RETURN for /rescue/sh"

[aki-k]

This is the rescue shell I get if I boot the qemu vm with -m 2048:

[grahamperrin]

Incidentally:

cannot read termcap database;
using dumb terminal settings.

– this aspect of /rescue/sh is also seen when intentionally preferring single user mode for a live boot.

#128 and for reference, helloSystem/docs@1770efc ▶ the note under https://hellosystem.github.io/docs/developer/boot.html#boot-into-verbose-single-user-mode

(This rescue shell experience differs significantly from what some users of FreeBSD might think of as a traditional rescue shell.)

[konfou]

Checking the files, there's already a message in overlays/ramdisk/init.sh#L103-L112 that runs on boot but doesn't show up because all messages are suppressed unless boot_mute='NO'(ref). A simple way then is to show that message on console regardless of what value boot_mute has. The relevant lines can be modified to

  echo "Live system requires 4GB of memory for memdisk, and operation!" >/dev/tty
  echo "Type exit, and press enter after entering the rescue shell to power off." >/dev/tty

[probonopd]

Thank you very much @konfou - trying it now.