tools: imx8m_image: Change source path for DDR firmware to build dir

The DDR firmware binaries are not part of the U-Boot source code, so
we should look for them in the build directory, where they need to be
copied to before building U-Boot.

The ATF binary is already fetched from the build directory, but the
README files for the i.MX8M EVKs claim that it needs to be copied to
the source directory (which is still true for in-tree builds, but not
in general). Therefore we also fix the READMEs to use the build
directory as the correct location for all additional binary files.

Sined-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>