UBUNTU: [Packaging] use ubuntu-core-initramfs to create initrd.img

Remove initramfs-tools from Build-Depends, and use ubuntu-core-initramfs
to create initrd.img.

Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
(backported from commit af1e3247084291aad7fbd47e80a471c3bae591fe)
(UBUNTU: denver: use ubuntu-core-initramfs to create initrd.img)
(source: https://git.launchpad.net/~canonical-kernel/denver/+git/linux-denver

)
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>