net: openthread: improve shell output

Following the introduction of the new shell, the OpenThread shell
wrapper is not well integrated. It outputs the answer to a command using
printk(), which is not necessarily using the same device than the shell
if "zephyr,console" does not match "zephyr,shell-uart".

OpenThread calls the otConsoleOutputCallback() function to output the
answer to the shell. It can provide a callback argument, but defined
when OpenThread is initialized, not when the command is submitted.
Workaround that by taking a copy of the shell argument in a static
variable and use it for the call to shell_fprintf(). It is not perfect
if two or more shells issue concurrent commands, but is already an
improvement compared to printk().

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>