BuildBootImage: Add secondstage image support

The AOSP bootimage format allows the use of a second stage image
however the BuildBootableImage function does not allows the "second"
optional argument. This patch adds the support of this argument.

Change-Id: I8ed9d9e56449945c2d42fc908269921c394f68c0
Signed-off-by: Benoit Fradin <benoit.fradin@intel.com>
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>