usb: class: Add USB mass storage class support.

This patch implements USB Mass Storage device Class,
which allows the  Zephyr target Board to appear
as a USB drive to a host.

This code would invoke disk_read() /disk_write()
hooks which are to be provided by the storage
layer.

The Mass Storage protocol state machine is based on
mbed's implementation. We augment it by adding Zephyr's
USB transport and Storage APIs, and offload disk ops
to a fiber context rather than in the USB irq context.

origin: https://developer.mbed.org/users/mbed_official/code/USBDevice/file/01321bd6ff89/USBMSD



Jira: ZEP-233

Change-Id: I8199598c76da20ab20012d81dac7615f6a366303
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>