scripts/package: snap-pkg target

Following in footsteps of other targets like 'deb-pkg, 'rpm-pkg' and 'tar-pkg',
this patch adds a 'snap-pkg' target for the creation of a Linux kernel snap
package using the kbuild infrastructure.

A snap, in its general form, is a self contained, sandboxed, universal package
and it is intended to work across multiple distributions and/or devices. A snap
package is distributed as a single compressed squashfs filesystem.

A kernel snap is a snap package carrying the Linux kernel, kernel modules,
accessory files (DTBs, System.map, etc) and a manifesto file.  The purpose of a
kernel snap is to carry the Linux kernel during the creation of a system image,
eg. Ubuntu Core, and its subsequent upgrades.

For more information on snap packages: https://snapcraft.io/docs/



Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>