We can use the same header length calculations for both imximage v1 and
v2. This addresses TODO comments about imximage v1 in the current code.

With this patch applied, *header_size_ptr in imximage_set_header() will
have the correct value for both imximage v1 and v2. This is necessary
for people wanting to add proprietary data behind the created imximage.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Cc: sbabic@denx.de