linux-yocto-tiny-kexecboot: fix build error (cpio not found)

As seen in the autobuilder the script generating the initramfs fails
because the .cpio is not found.

Match changes in the oe-core classes and redefine the INITRAMFS_TASK.
See commit 59a5f59 in oe-core "classes: Fix do_rootfs references"

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>