Switch the tx and rx buffer head/tail entries in the HardwareSerial
initialisation list so that they match the order the fields are defined
in. This fixes a compiler warning (repeated for each of the
HardwareSerial source files the header is used in).
Moreover, declaring pointers-to-registers as const and using initializer
list in class constructor allows the compiler to further improve inlining
performance.
This change recovers about 50 bytes of program space on single-UART devices.
See #1711
Cristian Maglie
Matt Robinson