initramfs-tools should allow building module-less generic initrds

[Oliver Grawert]

For the Ubuntu Touch images where modules are only loaded at a later point in the boot by firing up the android lxc container initramfs-tools should grow a MODULES=none config option which:

a) does not require a /lib/modules/$(uname -r) directory
b) does not try to copy any modules into the initrd

This will allow us to build fully generic initrds for systems that have all bits included in the kernel to find their root filesystems.
Additionally it will result in much smaller initrds on such systems.