kernelscripts: Make the buildkernel script more modular

Changes to make the buildkernel script more modular by
using functions, as well as introducing a new flag that
we case on to decide if we should use prebuilts or not.

Change-Id: I31d15b9ed0d2f6a5032d6f8cafa4cec6590d7d65
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>