cmd: elf: Prevent possible buffer overflow

cmd: elf: Prevent possible buffer overflow

In do_bootvx the environment variable 'bootdev' is fetched and copied
into a buffer without confirming that it will not overflow that buffer.
Use strlcpy to ensure that the buffer will not be overflowed.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>