coreboot/default.nix: reduce default loglevel

On kgpe-d16 coreboot has long been plagued with occasional boot
hangs somewhere deep within the PNP initialization code.  I'm still
hunting down the root cause, but for the moment reducing the logging
level helps.

Reducing the log level results in fewer fewer writes to the serial
port.  The serial port has a very finicky initialization sequence,
because it needs to be usable *before* the PNP initialization has
run.

src/coreboot/default.nix

@@ -5,7 +5,7 @@
 , config             ? throw "you must provide a coreboot .config file"
 , initramfs_image    # path to the initramfs `cpio` archive
 , iasl               ? null # a specific iasl to use, if needed
-, console_loglevel   ? "7"
+, console_loglevel   ? "6" # 8=SPEW, 7=DEBUG, 6=INFO
 , linux-command-line
 }:
 let