This adds a `hackrf_debug [-S|--state]` option, and the necessary
plumbing to libhackrf and the M4 firmware to support it.
The USB API and libhackrf versions are bumped to reflect the changes.
This removes the definition of the offset variable,
volatile uint32_t usb_bulk_buffer_offset = 0;
which is actually superfluous. This variable, along with its neighbour
usb_bulk_buffer_tx, is placed explicitly by the linker script. Its type
is defined by the declaration in usb_bulk_buffer.h.
There is no need to define it here, and doing so gives the misleading
impression that its initial value can be changed by modifying this line!
The initialization to zero never actually takes effect, because the
variable is not placed in the .data or .bss sections which are
initialised by the startup code.
The offset and tx variables are both set in set_transceiver_mode
before SGPIO streaming is started, so the M0 code does not use
them uninitialised.
This includes:
* Cmake clean up - thanks @Qyriad
* Windows binaries saved after each appveyor build
* A bump to the Visual Studio version that we use to build it
* An appveyor cygwin script for building firmware, it doesn't work but it seems like someone might pick it up and make it work, or blow it away if we switch to Travis firmware artefacts
By default, CMake assumes that the project is using both C and C++. By
explicitly passing 'C' as argument of the project() macro, we tell CMake
that only C is used, which prevents CMake from erroring out if a C++
compiler doesn't exist.
