imximage: Specify default IVT offset in IMX image

This patch adds BOOTROM_IVT_HDR_OFFSET at 0xC00. The BootROM expects to
find the IVT header at a particular offset in an i.MX image.

Defining the expected offset of the IVT header in the first-stage BootROM
image format is of use of later stage authentication routines where those
routines continue to follow the first-stage authentication layout.

This patch defines the first stage offset which later patch make use of.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Utkarsh Gupta <utkarsh.gupta@nxp.com>
Cc: Breno Lima <breno.lima@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>