new support for Raspberry Pi B+

[Dan Raymond]

I previously reported that I was able to get my 32GB SanDisk card to 
boot reliably after disabling the AutoSize feature.  All is not well, 
however, because I saw an SD card related panic while I was building pkg 
(about 40 minutes into it).  During the next reboot cycle it panicked 
again as follows:

mmcsd0: Error indicated: 1 Timeout
g_vfs_done():mmcsd0s2a[WRITE(offset=17725816832, length=32768)]error 
=mmcsd0: Error indicated: 1 Timeout 5
panic: No b_bufobj 0xcc41dfb8
KDB: enter: panic
[ thread pid 12 tid 100007 ]
Stopped at      $d:     ldrb    r15, [r15, r15, ror r15]!

This was using low speed for SD (25MHz).