lab2: simplify string printout

lab1_v2.S

@@ -2,26 +2,21 @@
 .cpu cortex-m4
 .thumb
 
+.macro BKPT_WRITE addr_src
+    mov r0, 0x4
+    mov r1, \addr_src
+    bkpt 0xAB
+.endm
+
 .section .data
     test_string: .asciz "Hello"
 
 .section .text
 
 lab1_v2:
-    sub sp, #6
+    push {lr}
-    ldr r0, =test_string
-    mov r1, #5
 
-    copy_loop:
+    ldr r1, =test_string
-    ldrb r2, [r0, r1]
+    BKPT_WRITE r1
-    strb r2, [sp, r1]
-    subs r1, #1
-    bpl copy_loop
 
-    mov r1, sp
+    pop {pc}
-    mov r0, 0x04
-    bkpt 0xAB
-
-    add sp, #6
-
-    mov pc, lr