Updated SDK concept proposal

Created by: jeremybennett

Review comments from Hugh O'Keeffe, Alex Fedorov and Olive
Zhao. Add a makefile to run spell checking.

Files changed in "program/Project Descriptions and Plans":

* SDK/Makefile: Created.
* SDK/custom.wordlist: Created.
* SDK/images/devkit.jpg: Created.
* SDK/images/diagrams.docx: Created.
* SDK/images/project-structure.odp: Deleted.
* SDK/images/project-structure.png: Deleted.
* SDK/images/refbrdsw.jpg: Created.
* SDK/images/sdk-overview.odp: Deleted.
* SDK/images/sdk-overview.png: Deleted.
* SDK/images/sdk.jpg: Created.
* SDK/sdk-project-concept.md: Updated with review comments,
including new diagrams.

Signed-off-by: Jeremy Bennett jeremy.bennett@embecosm.com