genimage.sh: fix calling from BR2_ROOTFS_POST_IMAGE_SCRIPT

genimage.sh: fix calling from BR2_ROOTFS_POST_IMAGE_SCRIPT

When called from BR2_ROOTFS_POST_IMAGE_SCRIPT, this script
ends up with following error:

Error: Missing argument

This is because, an extra positional argument is also passed
along with BR2_ROOTFS_POST_SCRIPT_ARGS. genimage.sh didn't
have support to parse positional and optional arguments
together.

Signed-off-by: Abhimanyu Vishwakarma <Abhimanyu.V@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>