AnHardt 4b44a23a36 Add TX-buffer for MarlinSerial
Similar to the current Arduino HardwareSerial
but with max. 256 byte buffer-size.

Deactivated by default.

The boards with AT90USB processor (USBCON) already use a TX-buffer.
..
Configuration.h Fix spelling in a config comment
Configuration_adv.h Add TX-buffer for MarlinSerial