fix: Add prerequisites and update docstrings (#58)

Essentially any target that uses `cargo-acap-build` needs this
prerequisite because it will try to use `acap-build` which fails if
there is no `LICENSE` file.
The cause of it being omitted is probably that I didn't think enough
about which targets needed it and since `make build` usually precedes
`make run` and `make test` without cleaning the repository between,
these targets appeared to work.

Docstrings are updated to reflect that, since switching from naked
`ssh` and `scp`, these verbs indirectly use `sshpass` to do password
authentication.

Makefile

@@ -95,8 +95,7 @@ stop:
 ## * The app is installed on the device.
 ## * The app is stopped.
 ## * The device has SSH enabled the ssh user root configured.
-## * The device is added to `knownhosts`.
-run:
+run: apps/$(AXIS_PACKAGE)/LICENSE
 	cargo-acap-build --target $(AXIS_DEVICE_ARCH) -- -p $(AXIS_PACKAGE)
 	acap-ssh-utils patch target/$(AXIS_DEVICE_ARCH)/$(AXIS_PACKAGE)/*.eap
 	acap-ssh-utils run-app \
@@ -117,8 +116,7 @@ run:
 ## * The app is installed on the device.
 ## * The app is stopped.
 ## * The device has SSH enabled the ssh user root configured.
-## * The device is added to `knownhosts`.
-test:
+test: apps/$(AXIS_PACKAGE)/LICENSE
 	# The `scp` command below needs the wildcard to match exactly one file.
 	rm -r target/$(AXIS_DEVICE_ARCH)/$(AXIS_PACKAGE)-*/$(AXIS_PACKAGE) ||:
 	cargo-acap-build --target $(AXIS_DEVICE_ARCH) -- -p $(AXIS_PACKAGE) --tests