tools: add support for setting the CSF into imximage

Add support for setting the CSF (Command Sequence File) pointer
which is used for HAB (High Assurance Boot) in the imximage by
adding e.g.

CSF 0x2000

in the imximage.cfg file.

This will set the CSF pointer accordingly just after the padded
data image area. The boot_data.length is adjusted with the
value from the imximage.cfg config file.

The resulting u-boot.imx can be signed with the FSL HAB tooling.
The generated CSF block needs to be appended to the u-boot.imx.

Signed-off-by: Stefano Babic <sbabic@denx.de>