zephyr: Switch to using FIXED_PARTITION_ macros

The FLASH_AREA_ macros, which have been using DTS node label property
to identify partitions, have been replaced with FIXED_PARTITION_
macros that use DTS node label to identify partitions.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>