imx: hab: Simplify the mechanism

The current mechanism is unnecessarily complex. Simplify the whole mechanism
such that the entire fitImage is signed, IVT is placed at the end, followed
by CSF, and this entire bundle is also authenticated. This makes the signing
scripting far simpler.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>