[BUG] Potentially empty Makefile variable used without checks

[DavidMCerdeira]

Describe the bug
ci.mk does not check if the CC variable has a value attributed to it. This may prevent the misra-check rule from executing correctly as it will not have a cc to invoke. In this case .cppcheck-types.cfg will not generate correctly.

To Reproduce
Steps to reproduce the behavior :

1. clone the https://github.com/bao-project/bao-libvmm/ repository, and enter branch wip/base-infra
2. run make -C ci/docker misra-check PLATFORM=qemu-aarch64-virt
3. misra check will fail due to bad .cppcheck-types.cfg file

Expected behavior
Misra check ci rule should either terminate successfully or output

Screenshots
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Additional context
Although this issue can be fixed by the projects that include bao-ci, bao-ci should perform sanity checks for uninitialized variables.