usb: gadget: Add boot marker for logging boot_kpi in sys node

This boot marker indicate the timestamp when USB device is
enumerated successfully and when USB device is entered from
suspend state to resume state. Boot marker is removed from
configfs.c file.

Change-Id: Ib0a97ba2702e94a87d15e7e94fa985c011d31908
Signed-off-by: Aniket Randive <arandive@codeaurora.org>