imximage: Fix imximage IVT bug for EIM-NOR boot

The load region size of EIM-NOR are defined to 0. For this case,
the parameter "imximage_init_loadsize" must be calculated.
The imximage tool implements the calculation in the "imximage_generate"
function, but the following function "imximage_set_header" resets the value
and not calculate. This bug cause some fields of IVT head are not
correct, for example the boot_data and DCD overlay the application area.

Signed-off-by: Ye.Li <B37916@freescale.com>