AT actions, i.e., AT commands that are neither setters nor getters, may
sometimes need to accept an optional argument delimited from the AT
command with a space, e.g., "AT$JOIN 1". This patch modifies the ATCI API
to support optional parameters in AT actions.

Previously, the only supported AT$AT syntax was AT$AT=AT... This can be
a bit confusing since, technically, the AT command is not a setter. It
just passes its argument to the LoRa modem. This patch adds support for
the following syntax:

AT$AT AT+....

That is, the AT$AT prefix and the actual AT command to be passed to the
LoRa modem are space-separated. The original setter syntax is also
retained.

When the buffer for custom AT commands isn't large enough to hold the
command, indicate an error to the caller rather than sending an
incomplete command to the LoRa module.

When an application invokes a custom LoRa command via AT$AT, pass the
response generated by the LoRa module to the upstream ATCI, on separate
lines prefixed with "$LORA: ". This feature provides applications using
the Tower Core module ATCI with full access to the LoRa module's ATCI in
a "tunneled" mode.

Also, only generate an "OK" response to AT$AT once the tunneled AT
command has been actually sent to LoRa module. If it cannot be sent,
e.g., because the command could not fit into an internal buffer, send an
"ERROR" instead.

This AT command can be used to enable or disable the forwarding of LoRa
responses and events over the ATCI. When set to 1, the responses
generated in response to custom AT commands and all asynchronous
notifications will be forwarded to the ATCI. Each such message will be
prefixed with "$LORA: ". When set to 0 (default), no LoRa responses and
asynchronous notification will be sent to the ATCI.

This command is meant to be used as a wrapper for custom LoRa AT
commands sent via AT$LORA that depend on asynchronous notifications, for
example:

AT$LORA>ATCI=1
AT$LORA AT$JOIN
OK
...
$LORA: +OK
$LORA: +EVENT=1,1
...
AT$LORA>ATCI=0

When two AT commands start with a common prefix, continue iterating
until you have found the right command.