Update device tree file and add stub for SYS_RT_SIGQUEUE

Signed-off-by: Harry Chen <i@harrychen.xyz>

kernel/src/arch/mipsel/board/malta/device.dts

@@ -9,7 +9,7 @@
 
 	chosen {
 		stdio = &uart2;
-		bootargs = "rust/sh";
+		bootargs = "sh";
 	};
 
     aliases { };

kernel/src/arch/mipsel/board/thinpad/device.dts

@@ -9,7 +9,7 @@
 
 	chosen {
 		stdio = &uart;
-        bootargs = "";
+        bootargs = "sh";
 	};
 
     aliases { };

kernel/src/syscall/mod.rs

@@ -287,7 +287,8 @@ impl Syscall<'_> {
             ),
             SYS_GETRANDOM => {
                 self.sys_getrandom(args[0] as *mut u8, args[1] as usize, args[2] as u32)
-            }
+            },
+            SYS_RT_SIGQUEUEINFO => self.unimplemented("rt_sigqueueinfo", Ok(0)),
 
             // custom
             SYS_MAP_PCI_DEVICE => self.sys_map_pci_device(args[0], args[1]),