MLK-10186-1 imximage: add fixed IVT offset support

Since from mx7, we use fixed IVT offset for all boot devices.
Introduce a new configuration CONFIG_IMX_FIXED_IVT_OFFSET for this.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Signed-off-by: Ye.Li <B37916@freescale.com>
(cherry picked from commit 88e0a3552b08627b18d98380a32dbafacb18854b)

Conflicts:
	tools/imximage.h