Switch to creating UEFI ESPs using loader.efi instead of boot1.efi.
Cope for the fact that laoder.efi, not being boot1, doesn't read
/boot.config by setting boot_serial and force the serial console.

Also add sysctl so we can display the boot method.

Provide a variable, do_boot1_efi, if you want to use boot1 for
testing. But since it's transient, it's just a variable and not
available on the command line.