test: refactor tests/simulator to compile more with cargo

* Build C files from build.rs script and remove "bitbox_merged" hack.
* Move cmake specifics out of build.rs.
* Rust functions exposed as a C api has been moved to the respective
  crates to remove circular dependencies.

Author: NickeZ

CMakeLists.txt

@@ -83,8 +83,8 @@ message(FATAL_ERROR
 endif()
 
 # Finds objcopy and other utilities for us
+include(CMakeFindBinUtils)
 if(CMAKE_CROSSCOMPILING)
-  include(CMakeFindBinUtils)
   find_program(CMAKE_SIZE "${TOOLCHAIN_PREFIX}size")
   message(STATUS "size: ${CMAKE_SIZE} ${TOOLCHAIN_PREFIX}size")
 endif()
@@ -389,7 +389,6 @@ if(CMAKE_CROSSCOMPILING)
   add_dependencies(doc rust-docs)
 else()
   include(CTest)
-  add_subdirectory(test/hardware-fakes)
   add_subdirectory(test/unit-test)
   add_subdirectory(test/simulator)
   if(COVERAGE)