When AT traces are enabled and very long AT commands are traced,
system can easily go to unwanted state. Therefore the length of the AT
traces is limited to DEBUG_MAXLEN (currently 60 characters).
This commit fixes read_hex_string, read_bytes and write to use this
limitter.
According to [3GPP-TS_24.008] the maximum size for Non-IP link MTU is 1358 octets to prevent fragmentation in the backbone network.
Therefore reduced the maximum size to follow the standard.
A new API `CellularDevice::clear()` to clean-up the modem to a default initial state.
Function is virtual so it can be overridden. The default implementation clears all PDP contexts,
but the the first one if that has APN defined as `nsapi.default-cellular-apn`.
CellularStateMachine calls `clear()` to clean-up the modem on initial `connect()`,
if the flag `cellular.clear-on-connect: true` is defined.