Fix for esp8266#4565 (rx fifo length), protect access to rx_buffer (esp8266#4568)

* Fix for esp8266#4565 (rx fifo length), protect access to rx_buffer

* Fix typo

* reworked to separate safe from unsafe functions, factorized some code, constness

* additional rework for uart_rx_fifo_available()

* swapped unsafe function definition order

* Remove static for overrun string

* Some shorthand for perf and readability

(cherry picked from commit 29580e8)

Author: devyte