This issue is specifically about an issue where 'sometimes' a box will fail to boot off of emmc (after having booted successfully previously).  The workaround is to boot once from SD card and then the emmc boot generally works fine again.   I am starting this thread as a central place to discuss this issue that has been reported by a few people that I know. @lgranie and @robertoenr both have reported this problem either recently or in the past and I see it myself frequently.