Improve sanity check

* Use 'git diff --name-only HEAD~1' to list changed files as
  "git show --name-only --format=''" doesn't work for pull requests.
* Add instructions on the installation of a specific version shellcheck.
* Add descriptions for the skipped pycodestyle and shellcheck codes.
* Skip the following shellcheck as 'which' is widely used and supported
  and busybox doesn't support 'command'.

  SC2230: which is non-standard. Use builtin 'command -v' instead.

Signed-off-by: Chase Qi <chase.qi@linaro.org>
diff --git a/sanity-check.sh b/sanity-check.sh
index 9174831..e72c7ed 100755
--- a/sanity-check.sh
+++ b/sanity-check.sh
@@ -5,4 +5,14 @@
     -g \
     -r build-error.txt \
     -p E501 \
-    -s SC1091
+    -s SC1091 SC2230
+
+# pycodestyle checks skipped:
+# E510: line too long
+
+# Shellchecks skipped:
+# SC1091: not following
+
+# Reason: 'which' is widely used and supported. And 'command' applets isn't
+# available in busybox, refer to https://busybox.net/downloads/BusyBox.html
+# SC2230: which is non-standard. Use builtin 'command -v' instead.