The override fields can be placed in the [Raspberry Pi] section inside
the systoad.conf file. They are designed to enable testing of the piboot
package with spread-like integration tests on hardware other than the
Raspberry Pi.

The documentation is updated but there is no code using the new fields
yet.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>