bootctl: write KERNEL_INSTALL_LAYOUT=bls and KERNEL_INSTALL_MACHINE_ID=…

bootctl: write KERNEL_INSTALL_LAYOUT=bls and KERNEL_INSTALL_MACHINE_ID=…

This is a natural extension of d6bce6e224: if we are installing sd-boot, we
want to use the sd-boot layout, so let's write the appropriate
KERNEL_INSTALL_LAYOUT setting. Effectively, if we do 'booctl install',
kernel-install will not autodetect the layout anymore.

And 357376d0bb added support for KERNEL_INSTALL_MACHINE_ID. We need to support
it here too. We both read it, so that we create the right directories, and also
write it if it wasn't written yet and we created some directories using it, so
that kernel-install that is executed later knows the machine-id that matches
the directories we crated.

The code is changed in some places to fail if we can't figure out the current
status. When installing the boot loader it's probably better not to guess.