boot: zephyr: Remove broken target config header feature

Remove broken target config header feature.
This has been broken for more than 6 years, and seeing as nobody noticed
it should be ok to remove it.

Broken because if(EXISTS path-to-file-or-directory) uses relative path.
From cmake documentation:
"Behavior is well-defined only for explicit full paths".

Secondly ${BOARD}.h does not account for changes introduced in hwmv2
where BOARD_QUALIFIER maybe should be handled as well.

Also, who will put their board config files here?

Signed-off-by: Joakim Andersson <joerchan@gmail.com>