[TESTS] Make autoexec command of basic firmware robust

Currently, the 'autoexec' command of basic firmware looks for command
list in Guest NOR flash at offset 0xFF000 terminating with NULL character.

The above approach is flawed because Guest NOR flash can contain garbage
non-ascii (on non-printable) characters which can cause 'autoexec' command
to detect command list with garbage characters.

This patch fixes 'autoexec' command of basic firmware by looking only for
ascii (or printable) characters at 0xFF000 offset of Guest NOR flash.

Signed-off-by: Anup Patel <anup@brainfault.org>