[ATCmdParser]: Align process_oob() to vrecv()'s newline handling

[betzw]

Description

The newline handling between methods ATCmdParser::process_oob() and ATCmdParser::vrecv() is not aligned among those in the sense that in ATCmdParser::process_oob():

• the newline characters '\n' and '\r' are not handled in a unified way
• the _output_delimiter is used to detect the end-of incoming messages, which is not correct for every kind of AT commands based modules (instead a kind of input delimiter would be required) and as a consequnce
• method ATCmdParser::process_oob() might fail to correctly recognize oobs under certain runtime conditions

Pull request type

• Fix
• Refactor
• New target
• Feature
• Breaking change

[AnttiKauppila]

@jarvte Please review and approve

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1395
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6303/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1049
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6303/

[mbed-ci]

Test : SUCCESS

Build number : 1183
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6303/1183