helper: command: drop last LF ('\n') from sequence of command_print()

helper: command: drop last LF ('\n') from sequence of command_print()

The OpenOCD commands produce their TCL text result through the
pair command_print() and command_print_sameline().
The latter is used to concatenate output in a single line.

At the end of a sequence of command_print(), the last LF is taken
as part of the command result, while it is not always needed, and
it is even annoying when the output of two commands needs to be
concatenate in a single line.

Using command_print_sameline() in place of the last call to
command_print() would solve the problem but it's quite expensive
in term of coding to fix all the existing commands.

Drop the last LF, if present.
Commands that would specifically need a LF as last char, can add
an extra LF at the end of the output.
Document this behavior in command.h.

Change-Id: I6757c20fbfce923dd393083146e8d5a1f3b790b4
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7471
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>