boot_serial: Fix Mynewt tests compilation

repos/mcuboot/boot/boot_serial/test/src/testcases/
    boot_serial_upload_bigger_image.c: In function
     ‘TEST_CASE_boot_serial_upload_bigger_image’:
repos/mcuboot/boot/boot_serial/test/src/testcases/
    boot_serial_upload_bigger_image.c:91:42: error:
     ‘Value8Bit’ undeclared (first use in this function)
91 | buf[payload_off + len - 2] = Value8Bit;
| ^~~~~~~~~

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>