simplify board API, adding stm32f3 discovery

- remove board header since it is not used

examples/device/cdc_msc_hid/ses/samd51/samd51.emProject

@@ -19,7 +19,7 @@
       arm_target_device_name="ATSAMD51J19"
       arm_target_interface_type="SWD"
       build_treat_warnings_as_errors="Yes"
-      c_preprocessor_definitions="__SAMD51_FAMILY;__SAMD51J19A__;ARM_MATH_CM4;FLASH_PLACEMENT=1;USE_SIMPLE_ASSERT;BOARD_METRO_M4_EXPRESS;CFG_TUSB_MCU=OPT_MCU_SAMD51"
+      c_preprocessor_definitions="__SAMD51_FAMILY;__SAMD51J19A__;ARM_MATH_CM4;FLASH_PLACEMENT=1;USE_SIMPLE_ASSERT;CFG_TUSB_MCU=OPT_MCU_SAMD51"
       c_user_include_directories="../../src;$(rootDir)/hw;$(rootDir)/src;$(asf4Dir);$(asf4Dir)/CMSIS/Include;$(asf4Dir)/include;$(asf4Dir)/config;$(asf4Dir)/hri;$(asf4Dir)/hal/include;$(asf4Dir)/hal/utils/include;$(asf4Dir)/hpl/port;$(asf4Dir)/hpl/gclk"
       debug_register_definition_file="ATSAMD51J19A_Registers.xml"
       debug_target_connection="J-Link"
@@ -100,6 +100,9 @@
       filter="*.c;*.h"
       path="../../../../../lib/segger_rtt"
       recurse="No" />
+    <configuration
+      Name="Metro M4 Express"
+      c_preprocessor_definitions="BOARD_METRO_M4_EXPRESS" />
   </project>
   <configuration Name="Metro M4 Express" />
 </solution>