usb: gadget: create a utility module for mass_storage

Converting to configfs requires making the f_mass_storage.c a module.

But first we need to get rid of "#include "storage_common.c".

This patch makes storage_common.c a separately compiled file, which is
built as a utility module named u_ms.ko. After all mass storage users are
converted to the new function interface this module can be eliminated
by merging it with the mass storage function's module.

USB descriptors are exported so that they can be accessed from
f_mass_storage.

FSG_VENDOR_ID and FSG_PRODUCT_ID are moved to their only user.

Handling of CONFIG_USB_GADGET_DEBUG_FILES is moved to f_mass_storage.c.
The fsg_num_buffers static is moved to FSG_MODULE_PARAMETER users, so
instead of using a global variable the f_mass_storage introduces
fsg_num_buffers member in fsg_common (and fsg_config).

fsg_strings and fsg_stringtab are moved to f_mass_storage.c.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>