fastboot: add the support of the second stage image

The AOSP bootimage format allows the use of a second stage image
however the fastboot boot and flash:row commands do not allow the
"secondstage" optional argument.  This patch adds the support of this
argument for both commands.

Change-Id: I00403a95d7460a00233841ba0fe0e8a69d78c7b4
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
(cherrypick of ecd28eff from AOSP; not sure how this got lost internally.)