configs/imxsabre: Fix U-Boot parallel build issue

configs/imxsabre: Fix U-Boot parallel build issue

Sometimes imximage throws the following error:

MKIMAGE u-boot-dtb.imx
Error: No BOOT_FROM tag in board/freescale/mx6sxsabresd/imximage.cfg.cfgtmp
arch/arm/imx-common/Makefile:91: 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.

This workaround has already been applied in mainline U-Boot:
http://git.denx.de/?p=u-boot.git;a=commit;h=b5b0e4e351e20a606de22db6a56ad6bc1e2aa8fd

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/59015347

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>