Add support for page offestting when issuing flash commands. The callee will not have any knowledge on this. The caller will use the information provided by the flash abstraction to decide on offsetting page numbers (if needed). This is mainly done to support cortex m0+ based SAM D21 devices, in which both the bootloader and the application reside in flash.
ChangeLog:
Define an API that returns the application's start page.
Use that API in places where a write/verify on flash is performed.Read operation is an exemption.
Remove hardcoding of page offsetting in writePage/readPage methods in Flash implementations