README.md: Update used version of avr-gcc
README.md: Change name of ATmega328 to ATmega328P
README.md: Change recommended fuse values

This changes a couple of fuse values, so they actually apply to the
ATmega328P as there are subtle changes compared to the ATmega168.
src: main.c: Refinements of Doxygen for wdt_init()
src: main.c: wdt_init(): Only clear WDRF flag

This makes sure that only the WDRF flag is being cleared, so other reset
sources could still be detected.
src: uart_protocol.c: _reset(): Fix deadlock situation

The UART functionality requires interrupts to be enabled, so cli() can only be
called once the output has been flushed.
src: uart_protocol.c: uart_protocol_handle(): Use strncmp() correctly"

Rather than using strlen() as argument for strncmp() use the buffer size.
bootloader: optiboot: Import version 6.0
bootloader: optiboot: Remove unused files
bootloader: optiboot: Add wordclock targets
bootloader: optiboot: optiboot.c: Remove unused defines
bootloader: optiboot: optiboot.c: Remove unused version section
bootloader: optiboot: optiboot.c: Remove unused OPTIBOOT_CUSTOMVER
bootloader: optiboot: Makefile: Fix section start for BIGBOOT
bootloader: optiboot: optiboot.c: Start application after watchdog reset
bootloader: optiboot: optiboot.c: Add support for Wordclock LEDs
bootloader: optiboot: Get rid of pin_defs.h
bootloader: optiboot: Get rid of boot.h
bootloader: optiboot.c: Remove unused conditionals
bootloader: optiboot.c: Refine description of optional defines