init: Don't set ro.serialno when androidboot.serialno is not set

This functionality is useful for improving boottimes on the ARC++
project. Without this change, ro.serialno would be set to the empty
string when androidboot.serialno was unset in the kernel commandline.

Bug: 62039211
Test: boot with androidboot.serialno unset and ensure ro.serialno is
unset

Change-Id: Iaee339dfa3f0c871e5e9c1fc0534347f2b3e8a07