Bootloader: fix it for 16 MHz ATmegas.

Now, this was an unexpected one: the Gen7 Arduino IDE Support
package's bootloader didn't work on 16 MHz chips and nobody
told me! Finally, Simon Fitzback is my hero. He told me about
the fault, I modified a Gen7 to 16 MHz found it myself. A really
small change.

The technical cause is, the single speed UART clock ist too inaccurate
for 16 MHz. Double speed is better. On 20 MHz, both clocls are
equally accurate, see ATmega's data sheet.
Arduino support: rebase library onto Arduino 1.0.3.