kernel-install: create the entry directory only if $BOOT/$MACHINE_ID exists

Things are currently fairly ugly in Fedora: we create $BOOT/$MACHINE_ID/$KERNEL_VERSION/,
and then 20-grub.install that is installed by grub2-common.rpm wants to remove that
directory before 50-dracut.install get a chance to run. 50-dracut.install
checks for the presence of that directory to decide where to install the
kernel. So let's make the creation of the directory conditional. Previous
commit changes bootctl install to create $BOOT/$MACHINE_ID, and this commit
makes kernel-install not create it. In effect, the entry directory will only be
created if 'bootctl install' or something else created the parent directory.

https://bugzilla.redhat.com/show_bug.cgi?id=1648907