config: Allow building selected kselftest targets
Enhance linux-base.yaml to provide more fine-grained control to higher config layers over how kselftests are built. Previously the higher layer could either enable or disable all kselftests to be built. Let's allow the higher layer to specify the target(s) it wants to build.
The following continues to build all kselftests:
- export BUILD_KSELFTESTS=true
The following continues to build no kselftests:
- export BUILD_KSELFTESTS=false
Anything else is interpreted as the TARGETS string:
- export BUILD_KSELFTESTS="mm arm64"
Signed-off-by: Ryan Roberts ryan.roberts@arm.com