mkinitramfs: Add MODULES=all option to add every module

Normally, adding every kernel module to the initramfs is undesirable. But it can be useful when trying to add hardware support for a new device, where we might not have all the necessary modules in the initramfs, don't know which ones we are missing, and need to boot the device to figure it out. Add an undocumented MODULES=all setting which adds everything.

(I'm not particularly attached to this, as I can do it with a hook)