qemuboot: Trigger write_qemuboot_conf task on changes of kernel image realpath

qemuboot: Trigger write_qemuboot_conf task on changes of kernel image realpath

The qemuboot.conf file contains the realpath of the kernel image
referenced by QB_DEFAULT_KERNEL. So, it must be recreated in case the
realpath of the referenced kernel image changes.

The variables KERNEL_IMAGE_NAME and KERNEL_IMAGE_BIN_EXT determine the
realpath of the kernel image relative to DEPLOY_DIR_IMAGE. Adding both of
them to the vardeps of the write_qemuboot_conf task triggers the
write_qemuboot_conf task in case the realpath of the kernel image
referenced by QB_DEFAULT_KERNEL changes.

Fixes: [YOCTO 15525]
Signed-off-by: "Weisser, Pascal" <pascal.weisser.ext@karlstorz.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>