distro: Refactor `debug-tweaks` usage for DISTRO_FEATURES

Inclusion of `EXTRA_IMAGE_FEATURES` complicates the build flow if
`IMAGE_FEATURES` is already present, instead we should append to it.

Therefore, refactor `cassini-sdk.inc` and `cassini-test.inc` to include
required `debug-tweaks` feature for distro.

This patch also fixes the typo.

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