I've got to the bottom of the issue with mSD on the BeagleBone.
Basically the GPIO_OMAP option has changed  in the 4.2 cycle and
this is used for card detection of the mSD card. Prior to this we
didn't have it explicitly in our config  so it was being selected
by something else and it was being built in rather than modular as
only with 4.2 can it now be built as modular, and as a result it came
up on our radar and was enabled as a module .
If I explicitly put this in the initrd it works fine, we have the
omap_hsmmc modular too, but it's not pulled in automatically as
there's no module dependy by omap_hsmmc on gpio_omap.
In the short term I've built it back into the kernel  as that's the
way it's been for likely ever so it'll appear in the next kernel build
If you want to use any of the other 4.2 kernels you can regenerate the
initrd withe the following line in the /etc/dracut.conf file