Add a linker script and example uart echo program

echo.S

@@ -0,0 +1,18 @@
+.section .text
+.globl _start
+
+.equ UART_DATA, 0
+.equ UART_STATUS, 1
+.equ UART_RX_READY = 0b10
+.equ UART_TX_READY = 0b01
+
+_start:
+    li a0, 0x1000
+
+loop:
+    lbu t0, UART_STATUS(a0)
+    andi t0, t0, UART_RX_READY
+    beqz t0, loop
+    lbu t0, UART_DATA(a0)
+    sb t0, UART_DATA(a0)
+    j loop

link.ld

@@ -0,0 +1,22 @@
+ENTRY(_start)
+
+SECTIONS {
+    . = 0x80000000;
+
+    .text : ALIGN(4) {
+        *(.text*)
+    }
+
+    .rodata : ALIGN(8) {
+        *(.rodata*)
+    }
+
+    .data : ALIGN(8) {
+        *(.data*)
+    }
+
+    .bss : ALIGN(8) {
+        *(.bss*)
+        *(COMMON)
+    }
+}