Zephyr already has a directory for storage API relevant headers.
Move disk_access.h header to include/storage where it fits better
structurally.
Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
Move USB MSC configuration to its own Kconfig file.
Align with other class designations and rename
mass_storage.c to msc.c
Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>