cmake: Set FX3 SDK path variable's default value from environment variable

The FX3 SDK installer creates the FX3_INSTALL_PATH variable, so there's
no need to hardcode a default path.
libbladeRF: Updated README.md

Added new backend option, removed documentation for items removed
in commit ee618d3e1.
host: Updated README.md

Updated backend flags and removed items from ee618d3e1.

Added additional Windows build information and the availability
of the Cypress backend.
libbladeRF: Added additional elaboration to SC16Q11 format
libbladeRF: [WIP] Began adding RX timestamp support (untested)
libbladeRF/test: Moved PRNG routine to common dir for use in other tests
libbladeRF: [WIP] Added preliminary timestamps test
libbladeRF: [WIP] Added GPIO bit definitions for timestamps
libbladeRF: [WIP] Added missing cases to switch statements
libbladeRF: [WIP] Added GPIO setup to sync_init
libbladeRF/test: [WIP] Fixed bugs in timestamp test
libbladeRF: [WIP] Fixes to logic errors in RX metadata handling
libbladeRF: [WIP] Added missing reset of message number
libbladeRF/test: Fixed logical errors with random gap in timestamp test
libbladeRF/test: [WIP] Moved sync config to main app param setup
libbladeRF/test: [WIP] Fixed logic when seeking across buffers, misc fixes
libbladeRF/test: [WIP] Fixed calculation of metadata samples left in buffer
libbladeRF/test: [WIP] Added missing message number reset
libbladeRF/test: [WIP] Added more test cases to test_rx_scheduled
libbladeRF: Added API definition for FPGA counter mode enable bit