tree-wide: use "_" rather than "-" as separator in kernel cmdline options

tree-wide: use "_" rather than "-" as separator in kernel cmdline options

Most of our kernel cmdline options use underscores as word separators in
kernel cmdline options, but there were some exceptions. Let's fix those,
and also use underscores.

Since our /proc/cmdline parsers don't distinguish between the two
characters anyway this should not break anything, but makes sure our own
codebase (and in particular docs and log messages) are internally
consistent.