Allow to force disable block based OTAs

In some cases the same device tree is used to support different
device variants which could have different partition layouts.
In order to use block based OTAs, it is required to set the size
of the system partition equals to the smallest one. However, this
means wasting space on those devices that have a larger system
partition. Add a new board flag to force disable block based OTAs.
In this way the entire block is used to create the file system and
no space is wasted.

Change-Id: I90882a3833a5a50e3c2b8affed39c049a61b18ac