How to handle the build dependencies of virtual targets. E.g. many recipes have build dependencies on virtual/libc, and we have both eglibc and uclibc to provide virtual/libc. Eglibc will be the default options but what if the user wants uclibc instead of eglibc. One possible way may be to treat this as part of the configuration PREPERRED_PROVIDER, and have the bitbake backend reparse the recipes based on the new configurations, but this way may take some time due to the reparsing.