lib & config: give shellcheck directions (to /dev/null for dynamic, to repo-relative path for static) for all sourced references

- in preparation for tightening the shellcheck severity level
  - it needs to be able to follow all sources; dynamic ones are ignored, static ones need root-relative prefix

lib/functions/compilation/uboot.sh

@@ -443,7 +443,9 @@ function compile_uboot() {
 
 function uboot_postinst_base() {
 	# Source the armbian-release information file
+	# shellcheck source=/dev/null
 	[ -f /etc/armbian-release ] && . /etc/armbian-release
+	# shellcheck source=/dev/null
 	source /usr/lib/u-boot/platform_install.sh
 
 	if [ "${FORCE_UBOOT_UPDATE:-no}" == "yes" ]; then