Isolate dynamic dependencies
Inspired by !4, this takes advantage (if you are using DDE30 or later) of a
new switch in amu 5.34 which causes dynamic dependencies to be stored in
(and later read back from) a file separate from the specified Makefile.
Tested on a BCM2835 ROM build. Afterwards, git status shows modified content only in ColourTrans (and that's just the deletion of a .gitignore file). All other non-silent submodule statuses are for untracked content, and these will gradually disappear as we update the relevant .gitignore files.