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>

target/initramfs/initramfs.mk

@@ -12,6 +12,8 @@ else
 INITRAMFS_TARGET:= #nothing
 endif
 
+TARGETS+=$(INITRAMFS_TARGET)
+
 $(INITRAMFS_TARGET) initramfs: host-fakeroot makedevs
 	rm -f $(TARGET_DIR)/init
 	ln -s sbin/init $(TARGET_DIR)/init