fs: Convert FCB to use sectors (not areas)

Prior to this commit: `struct fcb` contained a pointer to an array of
`struct flash_area`.  Each entry in the array represented a flash sector
that the fcb comprised.

After commit: `struct fcb` contains a pointer to an array of `struct
flash_sector`, not `struct flash_area`.

Rationale: The FCB needs to know which sectors it occupies for its
rotate operation.  However, the `flash_area` type is meant to represent
a collection of sectors, not a single sector.  `struct flash_area`
contains information that the fcb does not need (e.g., area ID and
device ID).  Furthermore, the flash_map API provides a means of
converting a flash area ID to an array of sectors
(`flash_area_get_sectors()`), but no way to convert to an array of
areas.

Signed-off-by: Christopher Collins <ccollins@apache.org>