Add extra flags to enable build caching

[marsfan]

See #296 for some discussion of the issue.

[Wenn0101]

@marsfan thank you for the PR! After you submitted this PR I re-evaluated the flags that are being used in our build. (I have been meaning to do this for a little while to reduce some of the less useful warnings during building).

When we build the mbed static library, it outputs some flags for use in our build. We created a process were we can ignore some of these flags, before storing them for use in the build. That is where the flags in -iprefix{runtime.platform.path}/cores/ @{build.variant.path}/mbed/.c-flags come from. -MMD was previously on this list of excluded flags. I removed it from the list of excluded flags to achieve the same result in this PR.

This is all a long winded way of saying that I liked your contribution, but I wanted to achieve it in a different way.
I want to make sure you get credit for this contribution, I'll see if I can figure out how to do that.
I am closing this PR in favor of the other one linked above.

[marsfan]

@Wenn0101 Great! As long as it caches the build files I'm happy.

[Wenn0101]

@marsfan thanks for the contribution. This change is live in the v2.0.4 release that went live today.