Kernel: Move linker variables definition to a header file

[mhermier]

The 'linker.ld' file has to define special external variable to map the memory layout so that memory management works properly.

Move every definitions (and remove the one that seems to be broken) to a single header in an effort to unify definitions.

[mhermier]

I see 3 advantages:

• Put the definition in one place (Don't repeat yourself)
• Remove the bogus definitions (Remove bogus and unused code)
• Enforce/documents that these definitions must exits for all supported platforms linkers scripts.

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[stale[bot]]

This pull request has been closed because it has not had recent activity. Feel free to re-open if you wish to still contribute these changes. Thank you for your contributions!