Default name for ramdisks to image.

If you build a ramdisk with ramdisk-image-create and forget to specify
-o, you end up with a .kernel and .initramfs file as the image outputs.
This commit defaults the $IMAGE_NAME value to image so you'd end up
with image.kernel and image.initramfs

Change-Id: I10f3ac9ad178c32119523e4da930070e3ade4f73
Merge "Default name for ramdisks to image."