linux-sysmocom: Catch-up with poky master

OE-Core seems to unpack the kernel sources into another
directory now (e.g. to share the sources with builds for
modules). Use the kernel.bbclass kernel_do_configure and
copy the .config to the build directory.

The kernel is now built in a dedicated build directory.

recipes-bsp/linux/linux-sysmocom_git.bb

@@ -34,9 +34,8 @@ FILES_kernel-image_sysmobts-v1 = ""
 
 require linux-tools.inc
 
-do_configure() {
-	install -m 0644 ${WORKDIR}/defconfig ${S}/.config
-	oe_runmake oldconfig
+do_configure_prepend() {
+	install -m 0644 ${WORKDIR}/defconfig ${B}/.config
 }
 
 # autoload defaults (alphabetically sorted)