Add check_for_file and check_for_tool functions to common.sh

These functions make it easy to do testing in scripts for required
files and tools and provide useful error messages when the files
or tools are not found.

BUG=chromium-os:11981
TEST=run cros_sign_bootstub in configurations that test all paths in
these functions.

Change-Id: Ia63cfabd1eecba6473331b756486745d71e6b699

Review URL: http://codereview.chromium.org/6591056

common.sh

@@ -647,3 +647,48 @@ prepare_test_image() {
   # From now on we use the just-created test image
   CHROMEOS_RETURN_VAL="$1/${CHROMEOS_TEST_IMAGE_NAME}"
 }
+
+# Check that the specified file exists.  If the file path is empty or the file
+# doesn't exist on the filesystem generate useful error messages.  Otherwise
+# show the user the name and path of the file that will be used.  The padding
+# parameter can be used to tabulate multiple name:path pairs.  For example:
+#
+# check_for_file "really long name" "...:" "file.foo"
+# check_for_file "short name" ".........:" "another.bar"
+#
+# Results in the following output:
+#
+# Using really long name...: file.foo
+# Using short name.........: another.bar
+#
+# If tabulation is not required then passing "" for padding generates the
+# output "Using <name> <path>"
+check_for_file() {
+  local name=$1
+  local padding=$2
+  local path=$3
+
+  if [ -z "${path}" ]; then
+    die "No ${name} file specified."
+  fi
+
+  if [ ! -e "${path}" ]; then
+    die "No ${name} file found at: ${path}"
+  else
+    info "Using ${name}${padding} ${path}"
+  fi
+}
+
+# Check that the specified tool exists.  If it does not exist in the PATH
+# generate a useful error message indicating how to install the ebuild
+# that contains the required tool.
+check_for_tool() {
+  local tool=$1
+  local ebuild=$2
+
+  if ! which "${tool}" >/dev/null ; then
+    error "The ${tool} utility was not found in your path.  Run the following"
+    error "command in your chroot to install it: sudo -E emerge ${ebuild}"
+    exit 1
+  fi
+}