[bsp, distro] Stop using c library to determine build type

Using the chosen c library to determine what should be included in the
cassini image versus any firmware only option is not helpful as this
limits users from selecting other c libraries in future and also adds
confusion when reading recipes.
To move away from this we have to set the DISTROOVERRIDES differently
for the firmware build to the main cassini image build.

Changelog: other
Signed-off-by: Drew Reed <Drew.Reed@arm.com>