CMake: fail for unsupported platforms

[axel-h]

Fail for unsupported platforms instead of doing something with potentially undefined or empty variables results in strange errors later that are a hard to understand.

[chrisguikema]

Does this not already happen in settings.cmake?

set(supported "tk1;tx1;tx2;exynos5422;qemu-arm-virt;odroidc2;zcu102")
if(NOT "${PLATFORM}" IN_LIST supported)
    message(FATAL_ERROR "PLATFORM: ${PLATFORM} not supported.
         Supported: ${supported}")
endif()

[axel-h]

True. But the check there and the setup here are two different places, so having the "else" here helps finding the places where new platforms must add code.

[axel-h]

But you have a point, we should have explicit checkes in settings.cmake also. Actually, moving all this to platform specific folder would be nicer, but that's a future step

[chrisguikema]

I agree about having settings and specific features imported from platform specific folders. That is so much nicer than having a load of if/else statements.

[chrisguikema]

I don't have a problem with merging this. Its definitely better to have settings/configuration options for each platform specifically called out, instead of running on assumptions.

[lsf37]

That looks good to me.

A somewhat related point (but not for action for this PR) is that it is easy to be confused between vm_minmal for Arm and minimal/minimal_64 for x86. In particular, when you try to provide something like x86 or pc99 to the vm_minimal app, you will be told that it's not supported and may well give up instead of ever finding the minimal app. We should probably give a hint for this in the error message there (and for Arm in the minimal apps).