tools: kwboot: Fix invalid UART kwbimage v1 headersz

tools: kwboot: Fix invalid UART kwbimage v1 headersz

Ensure that UART aligned header size is always stored into kwbimage v1
header. It is needed for proper UART booting. Calculation of headersz field
was broken in commit d656f5a0ee22 ("tools: kwboot: Calculate real used
space in kwbimage header when calling kwboot_img_grow_hdr()") which
introduced optimization of kwboot_img_grow_hdr() function.

Fixes: d656f5a0ee22 ("tools: kwboot: Calculate real used space in kwbimage header when calling kwboot_img_grow_hdr()")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>