Handle devices whose flash implementations cannot copy flash to flash

[multiplemonomials]

Currently, it seems like most Mbed devices are not capable of copying one location in flash memory to another in FlashIAP. This means that it has to get buffered, and the logical place to do this is in the FlashIAP class itself. This class should detect if a source region of memory falls inside the flash and, if so, buffer it in RAM.

Optional extra credit: adding a some sort of capability which indicates whether a device can copy flash to flash natively, so that the new code is only activated when actually needed.