[coreboot] Building 4.5 from release sources

[Trammell Hudson]

When I build coreboot 4.5 from the release sources it is necessary
to download the coreboot-blobs-4.5.tar.xz file and it looks like there
might be a dependency now on the 3rdparty/vboot tree as well since
cbfs.h includes vb2_api.h:

https://github.com/coreboot/coreboot/blob/master/src/commonlib/include/commo...

When building from git it checks out the correct submodule, so this is
only a problem for me with releases.  Is there an official release tar
for vboot sources?  I don't see one in https://www.coreboot.org/releases/.