destiny: using a bit field for tcp_header->data_offset is more convenient
In the current implementation the data offset is coded into an uint8_t.
Of this uint8_t only 3 bits apply for the data offset.
The remaining bits represent reserved flags for future use.
However, a proper bit masking is forgotten in order
to obtain the data offset part of this uint8_t.

Therefore, defining this uint8_t as a bit field allows a more convenient
method of access.
Merge pull request #1302 from cgundogan/fix_tcp_header_data_offset

destiny: using a bit field for tcp_header->data_offset is more convenient
Initial commit of Arduino Mega2560
strip functions in atmega2560/periph and atmega_common/thread_arch.c down to prototypes

Add custom errno.h since avr-libc doesn't include one
rename board_init to main, main to o_main (inside hello-world/main.c). Fix
Makefile.include:

UART works within RIOT codebase, :)
Develop: fix invocation of avrdude
arduino-mega2560: cleanup, add interrupt driven uart communication
SQUASHME: return examples/hello-world/main.c to original state
Arduino Mega2560: Make reset_handler entry point and move ISR to startup.c

    Create .init7 and .init8 sections to circumvent call to main
    Move ISR for UART0_RX to startup.c
    Some cleanup
Arduino Mega 2560: remove instrumentation from board_init()
Arduino Mega 2560: move logic from board_init() to reset_handler(), cleanup
Arduino Mega 2560: cleanup, remove unnecessary delays, add catchall loop for
debugging
Arduino Mega 2560: remove dummy main function and set entry point to
reset_handler to fix linking
Arduino Mega 2560: add Macros to get and set the global interrupt flag
Arduino Mega 2560: add atomic_arch_set_return to atomic_arch.c
Arduino Mega 2560: add arch specific implementation of functions to enable, disable and set the global interrupt flag, missing: function to see if in ISR
Arduino Mega 2560: Remove headers from SAM3X8E
Arduino Mega 2650: cleanup (changed references to samxxxx or portega)
Arduino Mega 2560: fix trailing white space
Arduino Mega 2560: further cleanup/license adjustments