Tree:

meta-bebot/
          recipes-kernel/
                         linux/
                              linux_YOUR_ACTUAL_VERSION_NUMBER.bbappend
                              files/
                                    defconfig

=== The bbappend itself, on my system linux-imx_3.0.35.bbappend
FILESEXTRAPATHS_prepend := "${THISDIR}/files:"

PRINC := "${@int(PRINC) + 7}"

SRC_URI += "file://defconfig"

do_configure_append() {
	cp ${WORKDIR}/defconfig ${S}/.config
}

=== The defconfig file, in the files subdir
CONFIG_ARM=y
CONFIG_HAVE_PWM=y
CONFIG_SYS_SUPPORTS_APM_EMULATION=y
CONFIG_HAVE_SCHED_CLOCK=y
CONFIG_GENERIC_GPIO=y
# CONFIG_ARCH_USES_GETTIMEOFFSET is not set
CONFIG_GENERIC_CLOCKEVENTS=y
CONFIG_GENERIC_CLOCKEVENTS_BROADCAST=y
...
### And so on