Added wildcard to mmcblk/loop if statement

This wound me up no end until I realised it was only checking for mmcblk0. My chromebook has mmcblk0 in use so the sd card was showing up as mmcblk1. It's also not inconceivable for people to have more than one SD card plugged in at once.
Replaced if with case

Because bash is the devil ;)
Merge pull request #2528 from rantpaste/create_sdcard-bug

Added wildcard to mmcblk/loop if statement