board: amlogic: fix buffler overflow in seria, mac & usid read

While meson_sm_read_efuse() doesn't overflow, the string is not
zero terminated and env_set*() will buffer overflow and add random
characters to environment.

Acked-by: Viacheslav Bocharov <adeep@lexina.in>
Link: https://lore.kernel.org/r/20240320-u-boot-fix-p200-serial-v2-1-972be646a301@linaro.org


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>