utils/pb-console: Trap SIGTERM on boot

utils/pb-console: Trap SIGTERM on boot

On kexec all processes will be sent a SIGTERM and SIGKILL. By default
there are messages on the console alerting the user to this, however in
some implementations these messages are disabled. This can have the
effect of the UI seemingly exiting to the shell on boot and hanging for
a short while before the kexec actually jumps into the next kernel.
Trap the SIGTERM sent to the pb-console parent process and print a short
message to the screen instead of momentarily dropping to the shell and
printing the usual help messages.

While here also cleanup the shell help messages below which are now
handled in the shell's .shrc file.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>