[BOOTSCRIPT][UBOOT] fix bootscript and add uboot addons

In the boot.env has been added the root_load for virtio which allows to set correctly the root path to /dev/vda* and root_mount has been changed such that depending on what devtype is used the root_load is set for mmc or virtio. To set these correctly has been added a suffix that depends on mmc. Indeed, it adds suffix = p, this because the root should be something like /dev/mmcblkp in which p represent the partition used. In the boot_clea-os.scr.in has been added the run for set_root_suffix to set "p" when mmc is detected as running block. Furthermore, to make sure that the build works the partition_tool_ext must be setted in UBOOT_ADDONS before building in the u-boot-common.inc. Signed-off-by: carlo carlo.ferriolo@seco.com