target/initramfs: actually build initramfs target

Currently, BR2_TARGET_ROOTFS_INITRAMFS doesn't actually do anything,
as it doesn't alter $(TARGETS)

This change hooks up the TARGETS variable to INITRAMFS_TARGET (which
is only set if BR2_TARGET_ROOTFS_INITRAMFS=y), so that the initramfs
list is built.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>