Build one osmo-trx binary for each device support enabled

Same way as we do in osmo-bts. After this commit, osmo-trx no longer
exists. Instead, osmo-trx-uhd and osmo-trx-usrp1 are generated based on
configure flags enabled.

A new flag --with(out)-uhd has been added to enable/disable build of
osmo-trx with UHD backend. It is left enabled by default to keep
compatibility with older build scripts. Binary with USRP1 backend must
still be manually enabled with --with-usrp1 flag.

Change-Id: Iea8c0d7434762713a53440d29bf3ebd84accb262
diff --git a/Transceiver52M/device/Makefile.am b/Transceiver52M/device/Makefile.am
index f0675b0..2788eaa 100644
--- a/Transceiver52M/device/Makefile.am
+++ b/Transceiver52M/device/Makefile.am
@@ -6,6 +6,8 @@
 
 if DEVICE_USRP1
 SUBDIRS += usrp1
-else
+endif
+
+if DEVICE_UHD
 SUBDIRS += uhd
 endif