ARM: imx: Include u-boot.img in u-boot-with-spl.imx if OF_SEPARATE=y

ARM: imx: Include u-boot.img in u-boot-with-spl.imx if OF_SEPARATE=y

The u-boot-with-spl.imx is a concatenation of SPL and u-boot.uim.
The u-boot.uim is u-boot.bin wrapped in uImage. In case OF_SEPARATE
is enabled, the u-boot.bin does not contain control DT for U-Boot,
and so u-boot.uim does not contain the DT, and so u-boot-with-spl.imx
does not contain the DT, and a system where u-boot-with-spl.imx is
written to offset 1024B to the start of storage no longer boots, as
it is missing DT.

In case OF_SEPARATE is enabled, u-boot.img contains both u-boot.bin
and the necessary DTs. Therefore, use u-boot.img instead of u-boot.uim
to generate u-boot-with-spl.imx when OF_SEPARATE is enabled.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Christoph Niedermaier <cniedermaier@dh-electronics.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>