Add Linux DFL backport driver

[pcolberg]

This is an early draft that substitues the Linux DFL backport driver for the Linux DFL kernel, to build against the existing kernel used in the Yocto image, e.g., the IOTG Yocto BSP kernel or the GSRD SoC FPGA kernel.

This complements OFS/linux-dfl-backport#115 and OFS/linux-dfl-backport#116.

[pcolberg]

To use the backport driver, modify GSRD's build.sh as follows:

diff --git a/build.sh b/build.sh
index 014db24..930e0d9 100755
--- a/build.sh
+++ b/build.sh
@@ -143,6 +143,7 @@ build_setup() {
 		echo -e "\n[INFO] Update bblayers.conf"
 		bitbake-layers add-layer ../meta-intel-fpga
 		bitbake-layers add-layer ../meta-intel-fpga-refdes
+		bitbake-layers add-layer ../meta-ofs
 		bitbake-layers add-layer ../meta-openembedded/meta-oe
 		bitbake-layers add-layer ../meta-openembedded/meta-python
 		bitbake-layers add-layer ../meta-openembedded/meta-networking
@@ -166,6 +167,8 @@ build_setup() {
 		echo 'PREFERRED_PROVIDER_virtual/kernel = "linux-socfpga-lts"' >> conf/site.conf
 		echo "PREFERRED_VERSION_linux-socfpga-lts = \"`cut -d. -f1-2 <<< "$LINUX_VER"`%\"" >> conf/site.conf
 		echo "KBRANCH = \"$LINUX_SOCFPGA_BRANCH\"" >> conf/site.conf
+		# Linux DFL
+		echo "MACHINE_ESSENTIAL_EXTRA_RDEPENDS += \"intel-fpga-dfl-mod\"" >> conf/site.conf
 		# U-boot
 		echo 'PREFERRED_PROVIDER_virtual/bootloader = "u-boot-socfpga"' >> conf/site.conf
 		echo "UBOOT_CONFIG:${MACHINE} = \"$UB_CONFIG\"" >> conf/site.conf

https://docs.yoctoproject.org/kernel-dev/common.html#incorporating-out-of-tree-modules