distro: Fix cassini-dev distro feature builds

When building with `cassini-dev` as DISTRO_FEATURE, the appropriate
image features are not turned on due to typo named as `cassini-devel`

This patch fixes the following:
* Remove `cassini-devel.inc`
* Add `cassini-dev.inc` to include `debug-tweaks` as IMAGE_FEATURE
* Update `cassini-distro-feature.bbclass` to reflect the above change

Changelog: bug
Signed-off-by: Vikas Katariya <vikas.katariya@arm.com>