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 <[email protected]>
Cc: Christoph Niedermaier <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: Peng Fan <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Ye Li <[email protected]>
Cc: uboot-imx <[email protected]>