Provide port-specific fetch of submodules

[dhalbert]

Fixes #8036.

• Rename top-level make fetch-submodules and remove-submodules to fetch-all-submodules and remove-all-submodules.
• In each ports/*, provide make fetch-port-submodules, which fetches only the submodules needed for that port.
• make fetch-port-submodules does not require BOARD to be set, but most port-specific Makefiles assumed that BOARD would always be set. They did not anticipate non-board-specific make targets. Fix that, by fixing how bad or missing BOARD is handled, and by guarding various actions in Makefiles to not do things if BOARD is not specified. New variables available now include VALID_BOARD and ALL_PORT_BOARDS.
• New script tools/fetch-submodules.sh. This could have been done solely in the makefiles, but it's less painful to use a shell script.

[jepler]

seems fine, didn't test. there's a low probability of folks invoking other targets in a port directory and not setting BOARD= but they can keep all the resulting pieces for themselves.

[dhalbert]

@jepler I found some bugs in the original intent of the PR -- I was so wrapped up in the other part I neglected to re-test. Tested again and make fetch-port-submodules now works.

[jepler]

Thanks! again, didn't test.