functions: Add ffs ep0 descriptor poll() to main loop
functions: Add ffs ep0 descriptor poll() to main loop

When ffs-based user function is created gadgetd has to
write descriptors and strings from service file and then
open at least ep0 and wait for incoming events.
When we receive ENABLE (or BIND depends on service config)
event we prepare environment for the service and call exec()

Change-Id: I4abd6b013aba0556f76e58214df19b2ef5204585
Signed-off-by: Krzysztof Opasiak <8064eda6f94b3817252b7d7649cf754592f63500@samsung.com>

gadgetd/gadgetd,gadgetd/gadgetd,gadgetd/gadgetd