general clean up

make some variables volatile
remove rx_buffer_next from header
more DEBUG output
fix warnings
coding conventions
change puts to DEBUG
rename end-context/stack
valgrind support
guard rtc syscall
native fix cpu_switch_context_exit
fix native ctxt switches

fix disableIRQ (remove context switch)
fix enableIRQ (make context switch delay-safe)
change interrupts-off signal-catch behaviour from error to warning
add context switch memory location warning
add syscall guard functions, wrap read/write
guard dlsym calls, wrap printf

gnu libc printf does not use write..
fix include induced warnings
move printf to uart
fix dlsym warning
posix: add semaphore header
remove sixlowpans own semaphore and use new riot semaphore in flowcontrol
rewrite mutex and semaphore