fix calc_rtt

closes #192
Merge pull request #598 from LudwigOrtmann/ping_rtt

fix calc_rtt
update native readme
fix paths in README.md
Merge pull request #600 from LudwigOrtmann/native_readme

update native readme
Merge pull request #599 from LudwigOrtmann/readme_md

fix paths in README.md
remove native cruft from cc110x_ng Makefile
Merge pull request #602 from LudwigOrtmann/cc110x_cruft

remove native cruft from cc110x_ng Makefile
Fix net_help include path
Merge pull request #603 from authmillenon/net_help_include

Fix net_help include path
Add USEPKG variable for Makefiles

Currently pkg/USING says one should use
`EXTERNAL+=$(RIOTBASE)/pkg/<pkg_name>` to enable PKG modules.
Using this line the PKG will be compiled but not linked.

This change adds a USEPKG variable to be used like
`USEPKG += <pkg_name>`, which looks less clumsy and gets the PKG linked
in the binary.
Use include path convention for PKGs
Add quadword math library

GCC implicitly calls functions like __adddi3 to handle arithmetics on
long long. Newlib depends on these functions in its printf() but
expects them to be present.

Since every function is defined in its own compiling unit, only the
function that are actually used will wind up in the binary.

There is no need for doxygen comments; you are not supposed to call
these functions directly.

Copied from http://ftp3.usa.openbsd.org/pub/OpenBSD/src/lib/libc/quad/
which is curtesy of the OpenBSD project (3 clause BSD license).
Add toolchain script for x86
[BLEEDING EDGES] first x86-multiboot commit
Dump
Implement getcontext and setcontext
Implement makecontext, hello-world works now!
Interrupt handling should be working, now
Don't put NOPs into the elf needlessly