message: Pass -Wno-sign-conversion to gcc.
  I believe this is no functional change: GCC's docs claim that
  sign-conversion is enabled by -Wconversion only for C code.
  GPC: sign-conversion
diff: @@ -45,7 +45,7 @@ build_unflags = -std=gnu11 -std=gnu++14
  [env:stm32]
  platform = ststm32
  board = custom_stm32
  -build_flags = ${env.build_flags} -Wconversion -Wno-error=register -mfpu=fpv4-sp-d16 -mfloat-abi=hard -DBARE_STM32 -Wl,-Map,stm32.map -Wl,-u,vectors -Wl,-u,_init
  +build_flags = ${env.build_flags} -Wconversion -Wno-sign-conversion -Wno-error=register -mfpu=fpv4-sp-d16 -mfloat-abi=hard -DBARE_STM32 -Wl,-Map,stm32.map -Wl,-u,vectors -Wl,-u,_init
  board_build.ldscript = boards/stm32_ldscript.ld
  build_unflags = -std=gnu11 -std=gnu++14
  extra_scripts = boards/stm32_scripts.py
  @@ -56,7 +56,7 @@ lib_extra_dirs =
  ${env.lib_extra_dirs}
  common/test_libs
  ; googletest requires pthread.
  -build_flags = ${env.build_flags} -DTEST_MODE -pthread -Wconversion
  +build_flags = ${env.build_flags} -DTEST_MODE -pthread -Wconversion -Wno-sign-conversion
  lib_deps =
  googletest
  ; This is needed for the googletest lib_dep to work. I don't understand why.
  