nvs-flash: Support for blobs larger than half of SPI Flash sector size

nvs-flash: Support for blobs larger than half of SPI Flash sector size

This change removes the earlier limitation of 1984 bytes for storing data-blobs.
Blobs larger than the sector size are split and stored on multiple sectors.
For this purpose, two new datatypes (multi-page index and multi-page data) are
added for entries stored in the sectors. The underlying read, write, erase and find
operations are modified to support these large blobs. The change is transparent
to users of the library and no special APIs need to be used to store these large
blobs.