bootloader: wordboot: Change default baud rate back to 9600

As the main application is using 9600 baud and the relative error is smaller at
this baud rate, the default value is switched back. This allows for Bluetooth
tranceivers to be used for both the main application as well as the bootloader.
bootloader: wordboot: Make BOOTLOADER_TIMEOUT_MS customizable in Makefile
bootloader: wordboot: wordboot.c: Refine Doxygen comments
bootloader: wordboot: wordboot.c: Define buff as real variable

As the .bss section is used by mcusr anyway, it is more intuitive to define
buff as real variable also, rather than some tricky define. While this
increases the generated code by a couple of bytes, it increases readibility
and is less dependent on the compiler placing things at the right location.
bootloader: wordboot: wordboot.c: Rename variable buff to page_buffer
README.md: Replace German PayPal button with International version
README.md: Mention github.com issue tracker
README.md: Remove reference to ATmega328P when talking about fuse settings
README.md: Refine sections about bootloader
bootloader: wordboot: Add reset_wordclock.sh script
bootloader: wordboot: README.md: Describe and document wordboot
Merge branch 'doc/bootloader'