onmetal: Replace boot_kernel's args

The console should be on ttyS4 and just in case blacklist mei.

The result looks like this:

```
label boot_kernel
  menu label boot_kernel
  kernel vmlinuz-boot_kernel
  append   ro noswap cros_legacy  root=LABEL=ROOT rootflags=subvol=root usr=gptprio: 8250.nr_uarts=5 console=ttyS4,115200n8 modprobe.blacklist=mei_me
```