build: disable generation of ramdisk images

Our build process works similarly to the image builder and doesn't support
the generation of initramfs-based images.

Fixes #616

include/config

@@ -1,5 +1,6 @@
 CONFIG_IMAGEOPT=y
 # CONFIG_PER_FEED_REPO is not set
+# CONFIG_TARGET_ROOTFS_INITRAMFS is not set
 CONFIG_DEVEL=y
 CONFIG_ALL_KMODS=y