Creation of IMAGE_FILE (if needed) not well implemented

[ckormanyos]

Some targets such as RPI-Zero and BBB use an image file to boot from SD card. The creation of $(IMAGE_FILE) --- if one is needed --- is, however, not well implemented in the build system. The rule for making an image file is in the target flags and this rule might use objcopy or dimilar. But those program symbols might not be defined at the time of including the target-specific build flags.

This issue is intended to clean up the support of the rule that creates the special image file on targets needing it.

When relevant, this issue addresses both the Win* as well as the *nix build.

[ckormanyos]

BBB needs its very simple SD card header.

Try to use standard simple (semi-portable) tools like the linker itself (ld, which can pack buffers) combined with objdump, possibly combined with xxd (and possibly also with sed or gawk if needed) to handle the very few hex manipulations required.