new support for Raspberry Pi B+

[Dan Raymond]

One small correction to my previous test results: with the 16GB 
Transcend card I *can* get it to boot reliably with the 
hw.bcm2835.sdhci.hs="0" hack.

To summarize, I see no improvement regarding SD card failures between 
r271779 and r273702.  Nor do I see any improvement between the previous 
commit to crochet and the current commit.  I do see a minor regression: 
without the sdhci.hs hack I used to get 50% boot failures on my 16GB 
Transcend and now I get 100% boot failures.