board/freescale: factorize genimage logic

board/freescale: factorize genimage logic

For the boards imx6ulevk, imx6sabresd, mx25pdk, mx51evk, mx53loco,
warpboard:
* Replace genimage.cfg with a common Freescale genimage template named
  genimage.cfg.template because they all use the same layout.  The only
  difference comes from the device tree blobs.
* Replace each post-image.sh script with a generic post-image.sh script
  which is able to generate the right genimage.cfg depending on:
** the image type (zImage or uImage) from BR2_LINUX_KERNEL_UIMAGE
** the device tree blobs from BR2_LINUX_KERNEL_INTREE_DTS_NAME
** the rootfs type (ext2, ext3 or ext4) from BR2_TARGET_ROOTFS_EXT2
* Fix the readme.txt files accordingly

Signed-off-by: Georges Savoundararadj <savoundg@gmail.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
[Thomas: remove handling of rootfs type, using rootfs.ext2 in all cases
is fine, rootfs.ext3 and rootfs.ext4 are just symbolic links to it.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>