In response to @Zuul's comment below his answer, usage of the
ln -sf /mnt/emmc /mnt/sdcard is not the appropriate way of dealing with it, because it's a soft-link and forcing it that way, can lead to some mis-behaviour of apps who are relying on the SDCard's mount point, by way of refusing to work with it, Play Store can fail in this regard.
The best solution is to use this:
busybox, explicitly bind the mount point to make
/mnt/emmc behave exactly like a SDCard, i.e:
/system/xbin/busybox mount -o bind /mnt/emmc /mnt/sdcard
Which will show up as a mount point in the proper way, if this was a soft-link it would end up like this -
/mnt/sdcard -> /mnt/emmc
Google Play store, as I mentioned has an internal checking mechanism to see if the directory is not a link (it will return a error of -22 IIRC).