init.qcom.usb.rc: Set serial string on init

Currently init.qcom.usb.sh is writing the serial number into
android sysfs entry. But during bootup, .rc file is writing
default composition before .sh gets executed. This results USB
driver sending default serial string to host due to port hopping
happens. Hence write ro.serialno to the sysfs entry on init once.

Also remove reading persist.usb.serialno which is required only for
NAND based devices as all future targets are emmc based devices.

CRs-Fixed: 659235
Change-Id: Iddb0875fe0aff9aa9d53850861cc54735e5006d7
Merge "init.qcom.usb.rc: Set serial string on init"