Adds rules to ignore untracked content

[CullenSharp]

Simply adds a rule so that untracked content is ignored in repos we don't own (i.e. omap image builder and beagle board device trees)

[ThirteenFish]

So this is fine, I don't hate it, but I like the idea better of cleaning up after ourselves in the build rules, so like the dtb make would have:

@cp $< $(REPO)/$(SRC_PATH)/
...
@rm  $(REPO)/$(SRC_PATH)/$<

and similarly symmetric cp/rms in the build_image script.

This effectively moves the rm up and out of the clean rule too.

This way we leave the submodules pristine and know we didn't accidentally leave any residual configs behind.

[CullenSharp]

Got it, I'll get to work on this ASAP. Gonna use this as an excuse to improve the makefile in general.