My tree: 
meta-bebot/
          recipes-kernel/
                         linux/
                              linux-gumstix-3.5.bbappend
                              files/
                                    defconfig


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My .bbappend file is :

FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
PRINC := "${@int(PRINC) + 7}"
SRC_URI += "file://defconfig"
do_configure_append() {
        cp ${WORKDIR}/defconfig ${S}/.config
}

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My defconfig file i changed this(CONFIG_VIDEO_VIVI=y) line:

# Miscelaneous helper chips
# CONFIG_VIDEO_THS7303 is not set
# CONFIG_VIDEO_M52790 is not set
#changed on Oct 31 1830hrs
CONFIG_VIDEO_VIVI=y
CONFIG_V4L_USB_DRIVERS=y
# CONFIG_VIDEO_AU0828 is not set