Fix test images kernel commandline arguments

By default, "cros_debug" should be included in the kernel commandline for
developer images. This change adds "cros_debug" to the kernel commandline
for test images, which are based on developer images but overwrite the boot
arguments.

TEST=Build test image, install on machine and grep "cros_debug" /proc/cmdline
BUG=chromium-os:17393

Change-Id: Ie0de11baf60a3a69a7fef0639247e2edae455ffb
Reviewed-on: http://gerrit.chromium.org/gerrit/3790
Tested-by: Arkaitz Ruiz Alvarez <arkaitzr@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>

build_image

@@ -869,7 +869,8 @@ mod_image_for_test () {
 
   # Now make it bootable with the flags from build_image
   "${SCRIPTS_DIR}/bin/cros_make_image_bootable" "${image_dir}" "${image_name}" \
-                                              ${crosbug12352_flag}
+                                              ${crosbug12352_flag} \
+                                              --force_developer_mode
 
   print_time_elapsed
 

image_to_vm.sh

@@ -227,7 +227,8 @@ dd if="${TEMP_ESP}"    of="${TEMP_IMG}" conv=notrunc bs=512 \
 #       to operate automatically below.
 ${SCRIPTS_DIR}/bin/cros_make_image_bootable $(dirname "${TEMP_IMG}") \
                                             $(basename "${TEMP_IMG}") \
-                                            --usb_disk /dev/sda3
+                                            --usb_disk /dev/sda3 \
+                                            --force_developer_mode
 
 echo Creating final image
 # Convert image to output format