Add link script for use with bootloader, and minimal build support

[terrillmoore]

The new MCCI bootloader (to be published soon) requires that the app live 20k offset into flash. We need the following:

1. Two link scripts, one for use with bootloader, the other without.
2. A menu item to select bootloader or not
3. An McciBootloader_AppInfo_t header (this can always be present)
4. Some place to put signature data -- this should be a 256-byte blob at the end of the image.
5. A toolflow that test-signs images for downloading using the test script. If debugging, it's obvious that we'll need to check and sign ELF images (which is going to require an probably-beneficial adjustment to the bootloader toolflow).

[terrillmoore]

Fix by #152