tools: kwbimage: Fix generating secure boot data image signature

tools: kwbimage: Fix generating secure boot data image signature

Secure boot data image signature is calculated from the data image without
trailing 4-bit checksum. Commit 77bf89a1f015 ("tools: kwbimage: Simplify
aligning and calculating checksum") unintentionally broke this calculation
when it increased payloadsz variable by 4 bytes which was propagated also
into the add_secure_header_v1() function. Fix this issue by decreasing size
of buffer by 4 bytes from which is calculated secure boot data image
signature.

Fixes: 77bf89a1f015 ("tools: kwbimage: Simplify aligning and calculating checksum")
Signed-off-by: Pali Rohár <pali@kernel.org>