Static layout: Generate tar.gz for static layout pnor

Add --image_type option to specify the image type of generated PNOR in
tarball.
Support "squashfs" and "static" image types.
Rename `generate-squashfs` to `generate-tar` to reflect that this util
is to generate a tarball instead of squashfs.

Example of usage:

    # Generate unsigned tarball for Witherspoon
    ./generate-tar -i squashfs witherspoon.pnor

    # Generate signed tarball with default key for Witherspoon
    ./generate-tar -i squashfs -s -- witherspoon.pnor

    # Generate unsigned tarball for Romulus
    ./generate-tar -i static romulus.pnor

    # Generate signed tarball with default key for Romulus
    ./generate-tar -i static -s -- romulus.pnor

Tested: Verify the generated tarball contains correct files.

Change-Id: Ibb66605663b28888bd643baf92d6e4bdbf60c206
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/generate-ubi b/generate-ubi
index 5c2b1b8..d54634c 100755
--- a/generate-ubi
+++ b/generate-ubi
@@ -4,7 +4,7 @@
 help=$'Generate PNOR UBI image from a PNOR SquashFS Tarball
 
 Generates a UBI, Unsorted Block Images, PNOR image from a PNOR SquashFS Tarball.
-The PNOR SquashFS Tarball is generated from the generate-squashfs script.
+The PNOR SquashFS Tarball is generated from the generate-tar script.
 
 usage: generate-ubi [OPTION] <PNOR SquashFS Tarball>...
 
@@ -44,7 +44,7 @@
 
 if [ ! -f "${tarball}" ]; then
   echo "Please enter a PNOR SquashFS Tarball."
-  echo "To generate PNOR SquashFS Tarball see generate-squashfs"
+  echo "To generate PNOR SquashFS Tarball see generate-tar"
   echo "$help"
   exit 1
 fi