kernel-install: fix detection of entry-token if $BOOT_ROOT is configured
    
    If $BOOT_ROOT is specified, but entry-token not, we'd skip the detection
    altogether, effectively defaulting to entry-token=machine-id.
    The case where $BOOT_ROOT was not specied, but entry-token was configured
    was handled correctly.
    This patch makes the handling of both symmetrical, i.e. will only set what
    wasn't configured.