Use the .ub for the recovery-ramdisk image

This changes the u-boot recovery ramdisj image from recovery-ramdisk.img.img
to recovery-ramdisk.ub

.ub is the preferred file extension for uboot ramdisk images.

This also changes the ambiguous BOARD_USES_MKIMAGE to BOARD_USES_UBOOT
which should be more descriptive.  This only affects the NookColor
right now.

Change-Id: I3fdc5cbd049fdd134fe311977d4fa36ed98e7840