storage: flash map: Add FIXED_PARTITION(label) macro

The commit adds FIXED_PARTITION(label) macro that allows to obtain
struct flash_area object for partition of given label.
The macro allows instantiation of partition at point of usage
and will be replacing need for defining flash map with all partition
entries.
Area obtained with the macro should not be passed to open, instead
flash_area_device_is_ready, basically equivalent of device_is_ready
should be called on the obtained pointer to check if area is ready
for use.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>