Uboot has some special code for rx51 for initializing one mmc device (do not remember if mmc0 or mmc1). Other one mmc device is initialized by NOLO. And sometimes (only sometimes on my n900 with low battery) I saw that one mmc device was not initialized (that one from NOLO). And because of that sometimes problem I was not able to properly write code for initialzing other one mmc device... And maybe you are seeing this NOLO problem and there is maybe other condition when mmc device is not initialized...

Other problem when uboot does not see bootmenu is when uboot fat driver cannot read files from MyDocs partition. And this can happen if some fsck tool change something in fat structure. Until now I still do not know which fat format structure is OK for uboot. What we know is that default vanilla eMMC fiasco image contains fat partition which is OK for uboot.