Major refactor in SDCard generation

Major refactor in SDCard generation

The generation of SDCard has been completely refactored to work out of
box; now to use it you have to set, e.g:

SDCARD_ROOTFS = "${DEPLOY_DIR_IMAGE}/${IMAGE_NAME}.rootfs.ext3"
SDCARD_SIZE = "128"
IMAGE_TYPES += "ext3 sdcard"

Now the sdcard has following layout:

 0  - 1M                  - reserved to bootloader and other data
 1M - BOOT_SPACE          - kernel
 BOOT_SPACE - SDCARD_SIZE - rootfs

All current supported machines on BSP will work out of box and
generate a 2GB SDCard with an ext3 rootfs on it.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>