USB: gadget: g_fs: code cleanup

This commit cleans the g_fs gadget hopefully making it more
readable.  This is achieved by usage of the usb_string_ids_tab()
function for batch string IDs registration as well as
generalising configuration so that a single routine is
used to add each configuration and bind interfaces.  As an
effect, the code is shorter and has fewer #ifdefs.

Moreover, in some circumstances previous code #defined
CONFIG_USB_FUNCTIONFS_GENERIC macro to prevent a situation
where gadget with no configurations is built.  This code removes
the #define form source code and achieves the same effect using
select in Kconfig.

This patch also changes wording and names of the Kconfig options.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>