scripts: lint-sh: ignore warnings about POSIX sh compatiblity

The following features are available in Busybox ash, so we don't need to
warn about them for runtime scripts:

- local keyword
- echo -n / -e
- String indexing

These warnings are new in shellcheck 0.7.2, which would otherwise fail
for various scripts.

scripts/lint-sh.sh

@@ -17,7 +17,7 @@ find package -type f | while read -r file; do
 	is_scriptfile "$file" || continue
 
 	echo "Checking $file"
-	shellcheck -f gcc -x -s sh -e SC2039,SC1091,SC2155,SC2034 "$file"
+	shellcheck -f gcc -x -s sh -e SC2039,SC1091,SC2155,SC2034,SC3043,SC3037,SC3057 "$file"
 done
 
 find scripts -type f | while read -r file; do