There are two issues causing the default configuration of 2015-08 not to boot correctly, both can be fixed by adding symbolic links in the root file system. Firstly systemd fails to mount a number of file systems as it is trying to use /usr/bin/mount to mount the file systems, which does not exit. This causes a number of service to fail preventing access to the system as /usr/sbin/sulogin cannot be found. To fix the problems the following two symbolic links were created. /usr/bin/mount -> ../../bin/mount /usr/sbin/sulogin -> ../../sbin/sulogin
We already have a patch in the queue to fix that problem: http://patchwork.ozlabs.org/patch/514750/
*** Bug 8331 has been marked as a duplicate of this bug. ***
The patch pointed by Vicente has been merged as of commit http://git.buildroot.net/buildroot/commit/system?id=c5bd8af65e50a51735eb112fed9cbe6337f14e06. This should fix this bug.