If you develop on windows and need cr/lf files, see this:

    https://git-scm.com/book/en/v2/Customizing-Git-Git-Configuration#_formatting_and_whitespace

    Git can handle this by auto-converting CRLF line endings into LF
    when you add a file to the index, and vice versa when it checks out
    code onto your filesystem. You can turn on this functionality with
    the core.autocrlf setting. If you're on a Windows machine, set it
    to true - this converts LF endings into CRLF when you check out code:

    $ git config --global core.autocrlf true

* HardwareSerial - availableForWrite() support

* fix indentation errors (tab vs space)

Added argument to HardwareSerial.begin to specify whether the logic levels of the UART rx and tx lines should be inverted (#719)

* Print error when serial number is invalid

* Move to the first check, change end by return

usefull for ESP8266 compatibility

Fixes: https://github.com/espressif/arduino-esp32/issues/270

NOTE: UART1's default pins can be used only when SPI Flash is DIO

thanks @Ladyada

many possible problems fixed in preparation for thread-safe