init: Fix serial number display for semc msm7x30 devices

Add proper check since semc msm7x30 bootloader exports
"serialno" in cmdline instead of the standard androidboot.serialno,
so init doesn't pick it

Signed-off-by: Michael Bestas <mikeioannina@gmail.com>

Change-Id: Ib4e715fafc64dd47fb667aeb63a7307334cca844