ensure files are rebuilt if any build flag is changed

Since build flags can be affected by choice in menuconfig we need to ensure that any affected files are rebuilt when this happens. The typical case will be when choosing different optimization level but there could be more obscure cases.

Possible solutions would be to:
* seek where these flags change and introduce rules that force a clean before make
* generate some kind of dependency between .o and their flags

I think this is a hard problem to get right but it is an important one to fix.