Because the shim has hardcoded paths for each distribution, it is
better to use the `grub-install --removable` argument to install into
`/EFI/BOOT` than to do it ourselves. This makes sure that all files
are present and loaded from correct locations.
The symptom before this change was that the shim could not find the
grubx64.efi file, which is because we were not installing it
correctly. So in fact the image would not boot, even though it would
boot after a Mender conversion.
Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>