This patch adds code to initialize the Global Descriptor Table. For
simplicity, the memory is organized following the flat model. Thus,
memory appears to Contiki as a single, continuous address space. Code,
data, and stack are all contained in this address space (so called
linear address space).
The macros to manipulate bits from segment descriptor and the
set_descriptor() helper are based on the ones described in [1].
[1] http://wiki.osdev.org/GDT_Tutorial
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