cleanup: Clarify use of flash layout CONFIGs

Based on feedback from programmers, it's not clear when
config_std_internal_flash should be used, and when non-standard
chip-specific layouts need to be defined. Add clarity here with the
following changes:

- Explain in-depth the one config that config_std_internal_flash should
  be used for.
- Move non-standard chip-level flash layout CONFIGs to their own new
  chip-level file, config_flash_layout. All chips should either include
  config_std_internal_flash.h OR define their own layout in their own
  config_flash_layout.

Functionally, this change is a NOP.

BUG=chrome-os-partner:23796
TEST=`make buildall -j`
BRANCH=None

Change-Id: I6037b68db9048d90fa2a2da4c9c9e09d1143fa68
Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/296527
Commit-Ready: Shawn N <shawnn@chromium.org>
Tested-by: Shawn N <shawnn@chromium.org>
Reviewed-by: Shawn N <shawnn@chromium.org>
6 files changed