Improve CI image build time

* Introduce cache for big downloads to improve image build time
* Utilize ccache for Linux kernel compilation to improve image build time

Signed-off-by: Anton Bondarenko <anton.bondarenko@arm.com>

Approved-by: Jens Elofsson <jens.elofsson@arm.com>
Approved-by: Jakub Sujak <jakub.sujak@arm.com>