util/abuild: check for PASSED_BOARDS before trying to show it

If no boards are tested by abuild, an error is currently shown because
no boards failed, but no boards passed either.  Account for this
possibility.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I097d3c728ca1acc652d5a1b7b49e57d01b0e513b
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69520
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

util/abuild/abuild

@@ -945,8 +945,10 @@ if [ "$recursive" = "false" ]; then
 		if [ "$exitcode" != "0" ]; then
 			failed=1
 		fi
-	else
+	elif [ -f "$PASSED_BOARDS" ]; then
 		printf "All %s tested configurations passed.\n" "$( wc -l < "$PASSED_BOARDS" )"
+	else
+		printf "No boards tested.\n"
 	fi
 fi