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
Build newlib with -flto
Less magic numbers for UART
Implement PIC
'Implement' GDT and PIT, fix PIC
Add page table (no #PF handler, yet)
Better assembly for ucontext
Dump
Page fault handler works now
Fix inISR() tracking
Add empty board.h