moved usrp1 and usrp2 fpga dirs into fpga subdirectory

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diff --git a/fpga/usrp2/top/u2_rev3_2rx_iad/README b/fpga/usrp2/top/u2_rev3_2rx_iad/README
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+This is a custom build for USRP2 FPGA.  It allows using a BasicRX or
+LFRX board and feed two independent, real signals.  In addition, instead
+of the CIC/HB decimator, which optimizes frequency response, it uses an
+integrate and dump decimator, which optimizes for time-domain impulse
+response.
+
+These changes have been made in dsp_core_rx.v:
+
+* A second DDC has been added, sharing a frequency register with
+  the existing DDC.
+
+* The output of the two DDCs are interleaved as I1 Q1 I2 Q2I ...
+  into the receive FIFO.  This limits the host configured decimation
+  to 8 intead of 4.  Use gr.deinterleave to recover the streams.
+
+* The ADCs are hardcoded:
+ 
+  RX_A ==>  DDC #1 I-input
+     0 ==>  DDC #1 Q-input
+  RX_B ==>  DDC #2 I-input
+     0 ==>  DDC #2 Q-input
+
+  Thus, the input mux has been disabled.
+
+* The CIC/HB decimator has been replaced by an integrate and dump at
+  the decimation rate.
+
+* To assist with meeting timing, the external RAM has been disabled.
+
+The basic application is to coherently sample two real IF streams and
+downconvert to baseband, while minimizing the impulse response duration
+of the resampling filters.