Fix save binary recipes

Issue raised here arduino/arduino-cli#355

Following the documentation
https://arduino.github.io/arduino-cli/platform-specification/#recipes-to-export-compiled-binary

Only one recipe.output.tmp_file and recipe.output.save_file should
be defined. The ".bin" only should be copied.
Now upload with arduino-cli works as "Export compiled Binary"

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

platform.txt

@@ -133,16 +133,14 @@ recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" {compiler.c.elf.f
 ## Create output (.bin file)
 recipe.objcopy.bin.pattern="{compiler.path}{compiler.objcopy.cmd}" {compiler.elf2bin.flags} {compiler.elf2bin.extra_flags} "{build.path}/{build.project_name}.elf" "{build.path}/{build.project_name}.bin"
 
-## Save bin
-recipe.output.tmp_file={build.project_name}.bin
-recipe.output.save_file={build.project_name}.{build.variant}.bin
-
 ## Create output (.hex file)
 recipe.objcopy.hex.pattern="{compiler.path}{compiler.objcopy.cmd}" {compiler.elf2hex.flags} {compiler.elf2hex.extra_flags} "{build.path}/{build.project_name}.elf" "{build.path}/{build.project_name}.hex"
 
-## Save hex
-recipe.output.tmp_file={build.project_name}.hex
-recipe.output.save_file={build.project_name}.{build.variant}.hex
+build.preferred_out_format=bin
+
+## Save binary
+recipe.output.tmp_file={build.project_name}.{build.preferred_out_format}
+recipe.output.save_file={build.project_name}.{build.variant}.{build.preferred_out_format}
 
 ## Compute size
 recipe.size.pattern="{compiler.path}{compiler.size.cmd}" -A "{build.path}/{build.project_name}.elf"