PPP packets from Serial on interrupt basis

# Basic Infos

- [x] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [x] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [ ] I have tested that the issue is present in current master branch (aka latest git).
- [ ] I have searched the issue tracker for a similar issue.
- [ ] If there is a stack dump, I have decoded it.
- [x] I have filled out all fields below.

#### Platform

- Hardware: ESP-07
- Core Version: #6680
- Development Env: Platformio
- Operating System: Ubuntu

### Settings in IDE

- Module: Generic ESP8266 Module
- Flash Mode: qio
- Flash Size: 4MB
- lwip Variant: v2 Higher Bandwidth
- Reset Method: ck
- Flash Frequency: 40Mhz
- CPU Frequency: 80Mhz
- Upload Using: SERIAL
- Upload Speed: 115200

### Problem Description

I talked with @d-a-v about implementing a mechanism using interrupts to handle data coming from Serial and send it directly to `ppp_input()` function. Can someone point me to the right direction?