bootloader: kousu-bootloader-dual

Make a dual MBR/EFI bootloader. There's not much in the way, except that  I've split kousu-bootloader-bios and kousu-bootloader-efi to save space on /boot when I know I will never use the other, and I can't recombine them under a metapackage because they both deploy the same hook.

I could move the hook to a shared dependency maybe?

Use kousu-bootloader for kousu-device-nigiri.