new support for Raspberry Pi B+

[Dan Raymond]

Luiz, I used crochet to create my own image (with r273619) and ran some 
tests.  Here are my results:

16GB Transcend card: I am getting consistent boot failures with mounting 
error (error 19).  The hw.bcm2835.sdhci.hs="0" hack has no effect on 
this image.  I tried 10 boot cycles and it failed every time.  
Previously I was getting 5 out of 10 boot failures without the hack and 
0 out of 10 boot failures with the hack.

32GB SanDisk card: I can boot consistently (I tried 10 boot cycles and 
it succeeded every time) unless I enable "option AutoSize". When 
autosizing is attempted I get the I/O read errors I mentioned earlier.  
I believe this is the same behavior I saw previously.