buildman: Add -F flag to retry failed builds

buildman: Add -F flag to retry failed builds

Generally a build failure with a particular commit cannot be fixed except
by changing that commit. Changing the commit will automatically cause
buildman to retry when you run it again: buildman sees that the commit
hash is different and that it has no previous build result for the new
commit hash.

However sometimes the build failure is due to a toolchain issue or some
other environment problem. In that case, retrying failed builds may yield
a different result.

Add a flag to retry failed builds. This differs from the force rebuild
flag (-f) in that it will not rebuild commits which are already marked as
succeeded.

Series-to: u-boot
Change-Id: Iac4306df499d65ff0888b1c60f06fc162a6faad8