selftests: lib: add config fragment for bitmap, printf and prime numbers tests

test_bitmap, test_printf and prime_numbers are expected to be built as
modules.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>