pyterm: fix exit behavior if twisted is not available
Merge pull request #1582 from N8Fear/fix_pyterm_twisted

pyterm: fix exit behavior if twisted is not available
cpu: atmega_common: Initial import
cpu: atmega2560: Initial import
board: arduino-mega2560: Initial import
tests: blacklist board arduino-mega2560:

    - tests/bloom: value too large for field of 2 bytes
    - tests/<other>: missing headers (time.h or sys/types.h)
test: add download of avr toolchain to .travis.yml
tests: blacklist tests/test_coap and examples that fail due to missing header files
boards: arduino-mega2560: cleanup of Makefiles
boards: arduino-mega2560: cleanup of periph_conf.h: remove left overs from
other board
boards: arduino-mega2560: fix indentation in errno.h
boards: arduino-mega2560: fix LED related defines
boards: arduino-mega2560: clean up LED init
cpu: atmega_common: move irq macros to irq_arch.c, fix indentation and doxgen
cpu: atmega2560: clean up Makefile
cpu: atmega2560: fix comment style
cpu: atmega2560: fix whitespace errors in header files and Makefile (periph)
cpu: atmega2560: move uart-related ISR to periph/uart.c, cleanup and comment
about custom vector table
cpu: atmega2560: fix newlines, whitespaces and typos in periph/timer.c
tests: fix blacklisting of arduino.mega2560 for test_struct_tm_utility and
unittests (missing time.h)