foc-arndale: use same context area location

Currently, on the Arndale platform, core uses a different thread context
area base address than the other tasks (0x20000000 vs. 0x40000000). This
is problematic, because core calculates the location of the UTCB area of a
new thread relatively to core's own thread context area base. So, the UTCB
area of non-core threads ends up in a virtual address range which is
outside of the task's thread context area and not marked as used in the
task's RM session.

With this patch, the same thread context area base address gets used in
core and in the other tasks.

Fixes #779.