Implements USRP2 peek() command, allowing arbitrary reads from the internal

Implements USRP2 peek() command, allowing arbitrary reads from the internal
Wishbone bus.  Minor fix for USRP2 sync_to_pps() (uses correct packet type.)

Example:

>>> from gnuradio import usrp2
>>> u = usrp2.source_32fc()
>>> u.peek(0x1234, 4) # Read four bytes at offset 0x1234 (code)
(185, 244, 253, 164)
>>>

The return value will be zero length upon error.

The read address must be 32-bit aligned, and only the lower 16 bits are
significant.  The length must be an integral multiple of 4 bytes. There is
currently a read limit of 176 bytes per read; to change requires some additional
firmware changes to allocate a larger reply packet.

WARNING: Trying to read from memory locations not serviced by RAM or by a
Wishbone peripheral may result in a hang requiring a USRP2 power cycle.  The
USRP2 internal memory map is documented in usrp2/firmware/lib/memory_map.h.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@10172 221aa14e-8319-0410-a670-987f0aec2ac5