When using socket stdio, add option to replay what has been written to
stdout while not connected (`-r`).
The implementation is to simply use the existing log file (which is
implicitly created when the option is used), and read from it until
EOF upon reconnect.
closes#476
wrap some libc functions that do system calls (terminal output)
wrap read/write with syscall guard
define real_read/write (next dynamic linker find for read/write)
guard system calls in remaining code
introduce native_internhal.h
throw out some debug statements that break things
clean up includes a bit
declare board_init in native_internhal.h
add -ldl to LINKFLAGS for cpu/syscalls
