board/common: fix rauc compatible regression

The new variable INFIX_COMPATIBLE, used for the RAUC compatible string,
is not properly evaluated on defconfigs where it is composed of another
variable, e.g.

    INFIX_COMPATIBLE="${INFIX_IMAGE_ID}"

Unfortunately, this is the default value for INFIX_COMPATIBLE, and so it
breaks all tier one defconfigs on the mainline branch since a1771ff.

Signed-off-by: Joachim Wiberg <[email protected]>

board/common/lib.sh

@@ -16,13 +16,22 @@ die()
 #      DISK_IMAGE_SIZE="512"
 #      etc.
 #
+# Nested variables, like INFIX_COMPATIBLE="${INFIX_IMAGE_ID}"
+# are handled by sourcing the file in a subshell.
+#
 # shellcheck disable=SC1090
 load_cfg()
 {
     tmp=$(mktemp -p /tmp)
+    (
+	. "$BR2_CONFIG" 2>/dev/null
 
-    grep -E "${1}.*=" "$BR2_CONFIG" >"$tmp"
-    .  "$tmp"
+	# Set *all* matching variables
+	set | grep -E "^${1}[^=]*=" | while IFS= read -r line; do
+            echo "$line"
+	done
+    ) > "$tmp"
 
+    .  "$tmp"
     rm "$tmp"
 }