cli: refactor kernel install code

The original plan was to support a KCI package type, which would
describe the package produced by KernelCI. However, that really just
boils down to 2 paths/URLs; one for the kernel image and aother for the
modules tarball. So instead we will just allow both paths/URLs to be
provided when the RAW package type is selected. This is more generic and
will allow straightforward usage with tuxmake and Shrinkwrap too.

Let's remove the KCI pkgtype, and refactor the install code to be able
to more easily extend it to supporting modules in the next commit.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>