Separate STM32F103R_bigtree_USB environment

platformio.ini

 platform_packages = tool-stm32duino
 extra_scripts     = buildroot/share/PlatformIO/scripts/STM32F1_SKR_MINI.py
 build_flags       = !python Marlin/src/HAL/HAL_STM32F1/build_flags.py
-  ${common.build_flags} -std=gnu++14
-  -DDEBUG_LEVEL=0 -DUSE_USB_COMPOSITE
+  ${common.build_flags} -DDEBUG_LEVEL=0 -std=gnu++14
+build_unflags     = -std=gnu++11
+lib_deps          = ${common.lib_deps}
+lib_ignore        = Adafruit NeoPixel, SPI
+src_filter        = ${common.default_src_filter} +<src/HAL/HAL_STM32F1>
+monitor_speed     = 115200
+upload_protocol   = stlink
+debug_tool        = stlink
+
+[env:STM32F103R_bigtree_USB]
+platform          = ststm32
+framework         = arduino
+board             = genericSTM32F103RC
+platform_packages = tool-stm32duino
+extra_scripts     = buildroot/share/PlatformIO/scripts/STM32F1_SKR_MINI.py
+build_flags       = !python Marlin/src/HAL/HAL_STM32F1/build_flags.py
+  ${common.build_flags} -DDEBUG_LEVEL=0 -DUSE_USB_COMPOSITE -std=gnu++14
 build_unflags     = -std=gnu++11
 lib_deps          = ${common.lib_deps}
 lib_ignore        = Adafruit NeoPixel, SPI