tools: kwboot: Simplify code for aligning image header

Expression (hdrsz % KWBOOT_XM_BLKSZ) is non-zero therefore expression
(KWBOOT_XM_BLKSZ - hdrsz % KWBOOT_XM_BLKSZ) is always less than value
KWBOOT_XM_BLKSZ. So there is no need to add another modulo. Also rename
variable `offset` to `grow` which better describes what is stored in
this variable.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>