imximage: Remove failure when no IVT offset is found

Sometimes imximage throws the following error:

  CFGS    board/freescale/vf610twr/imximage.cfg.cfgtmp
  CFGS    board/freescale/vf610twr/imximage.cfg.cfgtmp
  MKIMAGE u-boot-dtb.imx
Error: No BOOT_FROM tag in board/freescale/vf610twr/imximage.cfg.cfgtmp
arch/arm/mach-imx/Makefile:100: recipe for target 'u-boot-dtb.imx' failed

Later on, when running mkimage for the u-boot.imx it will succeed in
finding the IVT offset.

Looks like some race condition happening during parallel build when
processing mkimage for u-boot-dtb.imx and u-boot.imx.

A proper fix still needs to be implemented, but as a workaround let's
remove the error when the IVT offset is not found.

It is useful to have such message, especially during bring-up phase,
but the build error that it causes is severe, so better avoid the
build error for now.

The error checking can be re-implemented later when we have a proper
fix.

Reported-by: Breno Lima <breno.lima@nxp.com>
Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>