configure.ac: check boost only if USRP1 support is enabled
configure.ac: check boost only if USRP1 support is enabled

boost is only used in USRPDevice class.

It looks like it can be removed entirely quite easily, since only
boost::shared_ptr is used for 2 variables. That's left for somebody with
the device and willingness to test the changes.

Change-Id: I4c3fa3ff58fd552d0cb4c4cf2033615d84c07c96

ttsou/osmo-trx,ttsou/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx,osmocom/osmo-trx