use rt kernel 5.4.81-rt

stage3/05-install-rt-kernel/00-run.sh

@@ -0,0 +1,24 @@
+# get & extract pre-built rt kernel archive
+mkdir ${ROOTFS_DIR}/tmp/rt
+wget https://github.com/kdoren/linux/releases/download/5.4.81-rt45/kernel8-5.4.81-rt.tgz -O ${ROOTFS_DIR}/tmp/rt/kernel8-5.4.81-rt.tgz
+tar xzf ${ROOTFS_DIR}/tmp/rt/kernel8-5.4.81-rt.tgz -C ${ROOTFS_DIR}/tmp/rt/
+
+# copy kernel files to correct locations
+mkdir -p ${ROOTFS_DIR}/boot/rt/overlays
+cp -d ${ROOTFS_DIR}/tmp/rt/overlays/* ${ROOTFS_DIR}/boot/rt/overlays/
+cp -d ${ROOTFS_DIR}/tmp/rt/broadcom/bcm* ${ROOTFS_DIR}/boot/rt/
+cp ${ROOTFS_DIR}/tmp/rt/boot/kernel8_rt.img ${ROOTFS_DIR}/boot//rt/
+cp -dr ${ROOTFS_DIR}/tmp/rt/lib/* ${ROOTFS_DIR}/lib/
+rm -rf ${ROOTFS_DIR}/tmp/rt
+
+# append rt kernel options to /boot/config.txt
+cat >> ${ROOTFS_DIR}/boot/config.txt << EOF
+
+# boot with 64-bit realtime kernel
+arm_64bit=1
+kernel=rt/kernel8_rt.img
+#os_prefix=rt/
+overlay_prefix=rt/overlays/
+EOF
+
+sed -i "s/.*audio.*nice.*$/@audio   -  nice      -19/g" ${ROOTFS_DIR}/etc/security/limits.d/audio.conf