Fix compilation for rv32

kernel/src/arch/riscv32/board/virt/mod.rs

@@ -1,3 +1,6 @@
+#[path = "../../../../drivers/gpu/fb.rs"]
+pub mod fb;
+
 use crate::memory::phys_to_virt;
 
 /// Mask all external interrupt except serial.
@@ -16,3 +19,7 @@ pub unsafe fn enable_serial_interrupt() {
     UART16550.add(4).write_volatile(0x0B);
     UART16550.add(1).write_volatile(0x01);
 }
+
+pub fn probe_fb_info(_width: u32, _height: u32, _depth: u32) -> fb::FramebufferResult {
+    unimplemented!()
+}

kernel/src/arch/riscv32/mod.rs

@@ -1,12 +1,12 @@
 #[cfg(feature = "board_u540")]
 #[path = "board/u540/mod.rs"]
-mod board;
+pub mod board;
 #[cfg(feature = "board_rocket_chip")]
 #[path = "board/rocket_chip/mod.rs"]
-mod board;
+pub mod board;
 #[cfg(not(any(feature = "board_u540", feature = "board_rocket_chip")))]
 #[path = "board/virt/mod.rs"]
-mod board;
+pub mod board;
 
 pub mod compiler_rt;
 pub mod consts;