layer: use STAGING_KERNEL_BUILDDIR for defconfig

out-of-tree modules will not have .config in ${B}.
Check the defconfig in STAGING_KERNEL_BUILDDIR instead.

Add a bbnote, failing this test is not fatal if signing was disabled for
a kernel that inherited this class. More correctly this class should not
be inherited if the build kernel/modules are not intended to support
module signing.

Signed-off-by: Eric Chanudet <chanudete@ainfosec.com>

Author: Eric Chanudet

classes/module-signing.bbclass

@@ -32,7 +32,8 @@ def get_signing_key(d):
 SIGN_FILE = "${STAGING_KERNEL_BUILDDIR}/scripts/sign-file"
 
 fakeroot do_sign_modules() {
-    if ! grep -q '^CONFIG_MODULE_SIG=y' "${B}/.config"; then
+    if ! grep -q '^CONFIG_MODULE_SIG=y' "${STAGING_KERNEL_BUILDDIR}/.config"; then
+        bbnote "Kernel module signing deactivated in kernel configuration ${STAGING_KERNEL_BUILDDIR}/.config."
         return
     fi
     if [ -z "${KERNEL_MODULE_SIG_CERT}" ]; then