image: remove padding parameter from append-kernel/append-rootfs

image: remove padding parameter from append-kernel/append-rootfs

Using pad-to instead of passing the optional padding to append-kernel
or append-rootfs. It could be that the value of a variable is passed.
In case the variable is empty no error is thrown.

Furthermore the purpose of the extra parameter is hard to get without
reading the code.

Signed-off-by: Mathias Kresin <dev@kresin.me>