dependencies: add unzip

unzip is needed to uncompress at91bootstrap, so let's add it as a
dependency. This can be discussed, and if we think it shouldn't be a
new dependency, we can also build a host-unzip.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>