Change drive_prep() to look for the device instead of the mount

There is a subtle but annoying bug where using tab completion to give
the mount path adds a "/" at the end. When `grep` is run to check
if the image is mounted it searches with the trailing slash.

By looking for the partition instead we can avoid this small usability
issue which can cause confusion.
Strip trailing / from $IMGLOC

This reverts the previous change of checking for the mount point instead
of if $IMGLOC exists.

This change strips the trailing slash from $IMGLOC so the user does not
need to remember to remove the trailing slash manually.
Merge pull request #2 from mostlygeek/improve-usability

Change drive_prep() to look for the device instead of the mount