Could you try the next configuration?
[env:reprap_rambo]
monitor_speed = 115200
platform = https://github.com/platformio/platform-atmelavr.git
board = reprap_rambo
framework = arduino
build_flags = -w -Os -Wl,-u,vfprintf -lprintf_flt -lm -Wl,--gc-sections
lib_archive = false
[platformio]
src_dir = Firmware
build_dir = out