Add volatile software RTC to the TempDataLogger application if the dummy RTC mode is enabled.
Rename DS1307 driver files to a more generic RTC, as different RTC drivers may be added in the future.
First support for XMEGA in CDC bootloader
Button press to enter bootloader, whitespace
Fix jump to application for XMegas

The normal function-type evaluation does not work on XMega,
gcc-avr compiles it into a "load and execute far pointer" combination.

jmp 0 is what Atmel uses in it's bootloader assembly, and should be equivalent
to the function pointer.