Change boot button message to an independent sentence

[Neradoc]

resolves #7149
Removes a few messages mentioning the boot button, since that is the default anyway (if a boot button is defined).
The format is now:

You are in safe mode because:
Both buttons were pressed at start up.
To exit, please reset the board without requesting safe mode.

[dhalbert]

The Feather M0 RFM69 does not fit. There's not really anything obvious I can turn off, but I think the frozen library can be shrunk a bit without much difficulty.

[Neradoc]

So I changed the if (message != NULL) into a #if defined that I thought would have been optimized away anyway, so hopefully I didn't remove functionality in the process. Somehow it won us 16 bytes on the RFM69 from the previous run.

[dhalbert]

Great, that narrow change is what we needed! I'll merge to get this in to beta.4.

I looked at the size of the RFM69 ru build before this PR, and it had exactly 0 bytes remainining!

It would still be good to shrink that library a bit, since this will happen again.