board/freescale: Introduce UBOOTBIN in spl genimage templates

board/freescale: Introduce UBOOTBIN in spl genimage templates

For boards like imx7d-pico, u-boot.img changes to u-boot-dtb.img in
U-Boot v2019.07. Therefore, we need to pick u-boot.img or u-boot-dtb.img
depending on the board configuration.

Just like was done in commit 52344e556fa03 for the generic genimage
template, replace the U-Boot file name with UBOOTBIN, which is set
according to the BR2_TARGET_UBOOT_FORMAT_* option. We also need to set
the correct value for this variable for the u-boot.img and
u-boot-dtb.img cases.

While we're at it, rename the partition from 'u-boot-img' to 'u-boot',
to stay consistent with the generic genimage template.

Signed-off-by: Joris Offouga <offougajoris@gmail.com>
[Arnout: correct u-boot.img condition, rework commit log]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>