read android device serial number from sysfs node

read android serial number value from sysfs node
instead of kernel command line. Upstream boot managers
e.g., systemd-boot does not populate kernel command line
with parameter androidboot.serialno.

Reading serial number from sysfs node
/sys/devices/soc0/serial_number generalizes the approach
and makes it independent of bootloader that loads kernel.

Change-Id: I4998406b98aa928c852e897d1219c3c79e38b2ab