crosutils/make_factory_package.sh: support full SSD image as target outside chroot

When running outside chroot, we should use locate_cgpt + "$GPT" to invoke cgpt;
otherwise it may be not in system PATH.

For dealing with a true SSD image (over USB, for example) this CL also improved
data writing speed from <2M/s to 8M/s+ (over USB).

BUG=chromium-os:10531
TEST=./make_factory_package.sh \
  --factory=../build/images/x86-mario/latest/chromiumos_factory_image.bin \
  --release=../build/images/x86-mario/latest//chromiumos_image.bin \
  --diskimg=/dev/sdb

Also tested:
  --diskimg=blah # local file
  (chroot) --diskimg=/dev/sdd
  (chroot) --diskimg=blah

Change-Id: I647d97fe388f5b6a6223fa8cacfecdf00265f60c

Review URL: http://codereview.chromium.org/6261003

lib/cros_image_common.sh

@@ -185,6 +185,16 @@ image_partition_copy() {
     exit 1
   fi
 
+  # Try to use larger buffer if offset/size can be re-aligned.
+  # 2M / 512 = 4096
+  local buffer_ratio=4096
+  local bs=512
+  if [ $((dstoffset % buffer_ratio)) -eq 0 -a \
+       $((length % buffer_ratio)) -eq 0 ]; then
+    dstoffset=$((dstoffset / buffer_ratio))
+    bs=$((bs * buffer_ratio))
+  fi
+
   image_dump_partition "${src}" "${srcpart}" |
-      dd of="${dst}" bs=512 seek="${dstoffset}" conv=notrunc
+      dd of="${dst}" bs="${bs}" seek="${dstoffset}" conv=notrunc oflag=dsync
 }

make_factory_package.sh

@@ -110,9 +110,10 @@ prepare_img() {
   fi
 
   # Create GPT partition table.
+  locate_gpt
   install_gpt "${outdev}" 0 0 "${pmbrcode}" 0 "${force_full}"
   # Activate the correct partition.
-  cgpt add -i 2 -S 1 -P 1 "${outdev}"
+  sudo "${GPT}" add -i 2 -S 1 -P 1 "${outdev}"
 }
 
 prepare_omaha() {