Adds required Kconfig options for flash driver.
Adds FatFS filesystem for flash disk.
Fixes#14459.
Note: As USB MSC does not support multiple disks (see #14937),
only one (flash by default, modify CONFIG_MASS_STORAGE_DISK_NAME
to "RAM" for RAM disk) will appear to host.
Signed-off-by: Marcin Szymczyk <Marcin.Szymczyk@nordicsemi.no>
Added a warning that will abort build without disks.
Added a default configuration for nrf52840_pca10056.
Signed-off-by: Marcin Szymczyk <Marcin.Szymczyk@nordicsemi.no>