Meta: Integrate Shellcheck into Travis

lint-shell-scripts searches over the repository looking for shell
scripts. On those found, shellcheck is run against them. If any linting
fails print those warnings and exit with a non-zero exit code.

Run this script automatically in Travis.

Meta/lint-shell-scripts.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+set -e pipefail
+
+script_path=$(cd -P -- "$(dirname -- "$0")" && pwd -P)
+cd "$script_path/.."
+
+ERRORS=()
+
+for f in $(find . -path ./Root -prune -o \
+    -path ./Ports -prune -o \
+    -path ./.git -prune -o \
+    -path ./Toolchain -prune -o \
+    -type f | sort -u); do
+    if file "$f" | grep --quiet shell; then
+        {
+            shellcheck "$f" && echo -e "[\033[0;32mOK\033[0m]: sucessfully linted $f"
+        } || {
+            ERRORS+=("$f")
+        }
+fi
+done
+
+if (( ${#ERRORS[@]} )); then
+    echo "Files failing shellcheck: ${ERRORS[*]}"
+    exit 1
+fi