cassini-[bsp,config,distro]: Correct init manager for firmware builds

We see a build failures on master because:
- Cassini assigns the init manager to `systemd`
- The re-assignment back to mdev-busybox (firmware.conf) has no effect
- Enabling `systemd` now enables the `usrmerge` distro feature
- `usrmerge` affects the value of ${nonarch_base_libdir}
- some firmware components use ${nonarch_base_libdir} and some don't

We must fix the upstream firmware components for master, but in the
meantime, we should also ensure that firmware is being built with the
correct configuration.

Rather than using multiple hard assignments and relying on the order in
which bitbake applies them, use the mutually exclusive overrides
`cassini` and `cassini-firmware` to set the value of POKY_INIT_MANAGER

Note that the value of DISTROOVERRIDES when building the distro should
be...
- cassini
- cassini-parsec
- [cassini-dev]
- [cassini-sdk]
- [cassini-security]
- [cassini-test]

Whereas, when building firmware it should be...
- cassini-firmware

Signed-off-by: Adam Johnston <adam.johnston@linaro.org>