tools: imximage: refactor header length calculations for imximage v1

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