Repair initramfs dependancies after move to fs

The restructure for building root filesystems changed the target name
for the initramfs file, to build the file the trget is now
initramfs-root but the generated file is rootfs.initramfs

Signed-off-by: Will Wagner <will_wagner@carallon.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>