kernel.bbclass: use defconfig if it exists

It is common enough to want to apply a defconfig if the inheriting
recipe provides it. If the file exists in the $WORKDIR, copy it
over to ${S}/.config provided the target does not already exist.

This allows for recipes derived from kernel.bbclass to manage the
.config on their own, such as linux.inc.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>

meta/classes/kernel.bbclass

@@ -188,8 +188,12 @@ sysroot_stage_all_append() {
 	sysroot_stage_dir ${D}/kernel ${SYSROOT_DESTDIR}/kernel
 }
 
-
 kernel_do_configure() {
+	# Copy defconfig to .config if .config does not exist. This allows
+	# recipes to manage the .config themselves in do_configure_prepend().
+	if [ -f "${WORKDIR}/defconfig" ] && [ ! -f "${S}/.config" ]; then
+		cp "${WORKDIR}/defconfig" "${S}/.config"
+	fi
         yes '' | oe_runmake oldconfig
 }