KBDMUX_DFLT_KEYMAP is available for those of us using keyboard layouts different from the US/UK keyboard layout. This is very handy when running in single-user mode.
Add these lines to your custom kernel configuration file to have your keyboard layout preloaded.
options KBDMUX_DFLT_KEYMAP # SC #makeoptions KBDMUX_DFLT_KEYMAP=norwegian.iso # VT makeoptions KBDMUX_DFLT_KEYMAP=no
This only applies to the kernel once it has booted. The boot loaders are unaffected, and thus you need to know which keys on your keyboard corresponds to the keys on an US keyboard.
This is probably a strong sign that (U)EFI should be extended to initialise the correct keyboard layout after performing the POST.