linaro-media-create: needs to make image file a multiple of SD erase block size

Currently the linaro-media-create script creates image files (if invoked with --img_file) which are a multiple of the "cylinder size" in length. However, if the image is not an exact number of SD erase blocks in size then when it is passed to qemu the result is that Linux cannot access the half-erase-block at the end. The effect of this is that Linux complains that the partition is too large for the device:
[ 1.695709] mmc0: new SDHC card at address 4567
[ 1.703765] mmcblk0: mmc0:4567 QEMU! 3.99 GiB
[ 1.705596] mmcblk0: p1 p2
[ 1.722625] mmcblk0: p2 size 8241345 extends beyond EOD, truncated

and then fsck complains:

fsck from util-linux-ng 2.17.2
rootfs: The filesystem size (according to the superblock) is 1030168 blocks
The physical size of the device is 1030118 blocks
Either the superblock or the partition table is likely to be corrupt!

I'm going to attach a straightforward patch to linaro-media-create which forces the created image size to be a multiple of 256K (which is the largest permitted SDHC erase block size). Without this patch, running an image under qemu fails; with this patch it boots OK to a root prompt.

(images created with linaro-media-create --image_file 1005.img --rootfs ext3 --dev beagle --binary linaro-m-headless-tar-20100928-0.tar.gz --hwpack hwpack_linaro-omap3_20101005-37_armel_supported.tar.gz)

(NB that bug 626907 suggests that we ought to be aligning everything to the erase block size even on physical media; fixing that would fix this in passing but would be a more invasive set of changes.)