New Work Area Initialization API

[Sebastian Huber]

The work areas (RTEMS work space and C program heap) will be initialized now in
a separate step and are no longer part of rtems_initialize_data_structures().
Initialization is performed with tables of Heap_Area entries.  This allows
usage of scattered memory areas present on various small scale
micro-controllers.

The sbrk() support API changes also.  The bsp_sbrk_init() must now deal with a
minimum size for the first memory chunk to take the configured work space size
into account.