board/minnowboard: Unify Minnowboard and Minnowboard MAX boards

board/minnowboard: Unify Minnowboard and Minnowboard MAX boards

Given Minnowboard and Minnowboard MAX boards are very similar,
it's desirable to unify the support for them.

This commit does the following:

  1) Remove Minnowboard MAX's genimage.cfg, post-build.sh
     and post-image.sh. These are identical to Minnowboard's.

  2) Move Minnowboard MAX's linux config, and rename it.

     It would be lovely to have a single kernel config file.
     The kernel size penalty involved in adding support for
     all the peripherals on both boards is small enough to
     justify this.

     However, the original Minnowboard has some GPIO buttons
     that need to be registered by the kernel. This is not
     upstreamed, and hence we need to use the yocto v3.8 kernel
     to have this support.

  3) Rename each grub config to grub-{board}.cfg.

  4) Modify (the now unique) post-build script to use
     a different grub config, according to the board.

  5) Update both defconfigs, as per the above changes.

  6) Finally, update the readme.txt.

[Peter: mention MAX in readme title]
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>