fs/initramfs: fix initramfs support

On commit a2487758 TARGETS_ROOTFS was
introduced, however fs/initramfs/initramfs.mk was never updated, hence a
show-targets would be rootfs-initramfs with rootfs-cpio afterwards hence
never rebuilding the kernel with a proper cpio archive since TARGETS is
always before rootfs-* as stated in the commit description.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>