tests/qtest/boot-serial-test: Reduce for() loop in PL011 tests

Since registers are not modified, we don't need
to refill their values. Directly jump to the previous
store instruction to keep filling the TXDAT register.

The equivalent C code remains:

  while (true) {
      *UART_DATA = 'T';
  }

Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Fabiano Rosas <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>

Author: philmd

tests/qtest/boot-serial-test.c

@@ -70,18 +70,18 @@ static const uint8_t kernel_plml605[] = {
 };
 
 static const uint8_t bios_raspi2[] = {
-    0x08, 0x30, 0x9f, 0xe5,                 /* loop:  ldr     r3, [pc, #8]   Get &UART0 */
+    0x08, 0x30, 0x9f, 0xe5,                 /*        ldr     r3, [pc, #8]   Get &UART0 */
     0x54, 0x20, 0xa0, 0xe3,                 /*        mov     r2, #'T' */
-    0x00, 0x20, 0xc3, 0xe5,                 /*        strb    r2, [r3]       *TXDAT = 'T' */
-    0xfb, 0xff, 0xff, 0xea,                 /*        b       -12            (loop) */
+    0x00, 0x20, 0xc3, 0xe5,                 /* loop:  strb    r2, [r3]       *TXDAT = 'T' */
+    0xff, 0xff, 0xff, 0xea,                 /*        b       -4             (loop) */
     0x00, 0x10, 0x20, 0x3f,                 /* UART0: 0x3f201000 */
 };
 
 static const uint8_t kernel_aarch64[] = {
-    0x81, 0x0a, 0x80, 0x52,                 /* loop:  mov    w1, #'T' */
+    0x81, 0x0a, 0x80, 0x52,                 /*        mov    w1, #'T' */
     0x02, 0x20, 0xa1, 0xd2,                 /*        mov    x2, #0x9000000  Load UART0 */
-    0x41, 0x00, 0x00, 0x39,                 /*        strb   w1, [x2]        *TXDAT = 'T' */
-    0xfd, 0xff, 0xff, 0x17,                 /*        b      -12             (loop) */
+    0x41, 0x00, 0x00, 0x39,                 /* loop:  strb   w1, [x2]        *TXDAT = 'T' */
+    0xff, 0xff, 0xff, 0x17,                 /*        b      -4              (loop) */
 };
 
 static const uint8_t kernel_nrf51[] = {